libsframe/doc/sframe-spec.texi

\input texinfo       @c                    -*- Texinfo -*-
@setfilename sframe-spec.info
@settitle The SFrame Format

@copying
Copyright @copyright{} 2021-2026 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU General Public License, Version 3 or any
later version published by the Free Software Foundation.  A copy of the
license is included in the section entitled ``GNU General Public
License''.

@end copying

@dircategory Software development
@direntry
* SFrame: (sframe-spec).         The Simple Frame format.
@end direntry

@titlepage
@title The SFrame Format
@subtitle Version 3
@sp 15
@center @strong{*** DRAFT - NOT FOR DISTRIBUTION ***}
@center @today{}
@author Indu Bhagat

@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage
@contents

@ifnottex
@node Top
@top The SFrame format

@center @strong{*** DRAFT - NOT FOR DISTRIBUTION ***}
@center @today{}

This manual describes Version 3 of the SFrame file format.  SFrame stands for
Simple Frame.  The SFrame format keeps track of the minimal necessary
information needed for generating stack traces:

@itemize @minus
@item
Canonical Frame Address (CFA).
@item
Frame Pointer (FP).
@item
Return Address (RA).
@end itemize

The reason for existence of the SFrame format is to provide a simple, fast and
low-overhead mechanism to generate stack traces.

@menu
* Introduction::
* SFrame Section::
* Interpretation of SFrame FREs::

Appendices
* Generating Stack Traces using SFrame::

* Index::
@end menu

@end ifnottex

@node Introduction
@chapter Introduction
@cindex Introduction

@menu
* Overview::
* Changes from Version 2 to Version 3::
* Changes from Version 1 to Version 2::
@end menu

@node Overview
@section Overview
@cindex Overview

The SFrame stack trace information is provided in a loaded section, named
@code{.sframe}.  When available, the @code{.sframe} section appears in segment
of type @code{PT_GNU_SFRAME}.  An ELF SFrame section will have the type
@code{SHT_GNU_SFRAME}.

The SFrame format is currently supported only for select ABIs, namely, AMD64,
AAPCS64, and s390x.

A portion of the SFrame format follows an unaligned on-disk representation.
Some data structures, however, (namely the SFrame header and the SFrame
function descriptor index) have elements at their natural boundaries.  All data
structures are packed, unless otherwise stated.

The contents of the SFrame section are stored in the target endianness, i.e.,
in the endianness of the system on which the section is targeted to be used.
An SFrame section reader may use the magic number in the SFrame header to
identify the endianness of the SFrame section.

Addresses in this specification are expressed in bytes.  The use of term `data
word' in this document is colloquial; it should not be understood to correlate
with the architectural machine word or any specific hardware data width.

The rest of this specification describes the current version of the format,
@code{SFRAME_VERSION_3}, in detail.  Additional sections outline the major
changes made to each previously published version of the SFrame stack trace
format.

This document is intended to be in sync with the C code in @file{sframe.h}.
Please report discrepancies between the two, if any.

@node Changes from Version 2 to Version 3
@section Changes from Version 2 to Version 3
@cindex Changes from Version 2 to Version 3

The following is a list of the changes made to the SFrame stack trace format
since Version 2 was published.  Note that SFrame Version 2 had up to two
Errata.

@itemize @bullet
@item
Terminology improvements and renames for readability
 @itemize @minus
  @item Use the terminology `PC offset' in place of `Addr' for function start
  PC offset consistently.
  @item Make a distinction between SFrame FDE Type (e.g.,
   @code{SFRAME_FDE_TYPE_DEFAULT}, @code{SFRAME_FDE_TYPE_FLEX}) vs SFrame FDE PC
   Type (i.e., @code{SFRAME_FDE_PCTYPE_MASK}, @code{SFRAME_FDE_PCTYPE_INC}).
  @item Instead of using the term `info word', use a more precise term `info
  byte' in specification for the info bytes in SFrame FDE and SFrame FRE.
  @item Use term `data word' instead of `offset' to convey the functional role
  of the variable-length array of bytes trailing the SFrame FRE header.  With the
  introduction of flexible FDE type, the interpretation of those bytes is not
  always as an offset.
  @item Rename @code{SFRAME_FRE_OFFSET_<N>B} to @code{SFRAME_FRE_DATAWORD_<N>B}.
 @end itemize
@item
Reorganize the SFrame function descriptor entry into two distinct structures:
 @itemize @minus
  @item SFrame function descriptor index
  @item SFrame function descriptor attribute
 @end itemize
 Rename structure members as a consequence.
@item
Narrow the width of @code{sfda_func_num_fres} to @code{uint16_t} and remove
padding field @code{sfde_func_padding2}.
@item
Increase the width of the @code{sfdi_func_start_offset} to @code{int64_t}.  This
field is renamed from the @code{sfde_func_start_address} in SFrame Version 2
specification.
@item
Signal frames are marked with one bit in @code{sfda_func_info}.
@item
Addition of a new function info byte @code{sfda_func_info2} in SFrame function
descriptor attribute structure to store additional information about the stack
trace data for the function.
@item Reserve 5-bits for FDE types.  Define two FDE types: default FDE type
@code{SFRAME_FDE_TYPE_DEFAULT}, and flexible FDE type @code{SFRAME_FDE_TYPE_FLEX}.
@item
Define a new FDE type @code{SFRAME_FDE_TYPE_FLEX} to convey stack trace information for
specific cases, e.g., when CFA is non-SP/FP based, or when FP/RA recovery is
REG-based.
@item
An SFrame FDE of type @code{SFRAME_FDE_TYPE_DEFAULT} with no FREs is used to
indicate an outermost frame.
@item On s390x, use FDE type @code{SFRAME_FDE_TYPE_FLEX} to encode FP/RA
recovery from REG, instead of encoding DWARF register number in the SFrame FRE
variable-length data of FDE type @code{SFRAME_FDE_TYPE_DEFAULT}.
@end itemize

@node Changes from Version 1 to Version 2
@section Changes from Version 1 to Version 2
@cindex Changes from Version 1 to Version 2

The following is a list of the changes made to the SFrame stack trace format
since Version 1 was published.

@itemize @bullet
@item
Add an unsigned 8-bit integral field to the SFrame function descriptor entry to
encode the size of the repetitive code blocks.  Such code blocks, e.g., pltN
entries, use an SFrame function descriptor entry of type
@code{SFRAME_FDE_PCTYPE_MASK}.
@item
Add an unsigned 16-bit integral field to the SFrame function descriptor entry
to serve as padding.  This helps ensure natural alignment for the members of
the data structure.
@item
The above two imply that each SFrame function descriptor entry has a fixed size
of 20 bytes instead of its size of 17 bytes in SFrame format Version 1.
@item
[Errata 1] Add a new flag SFRAME_F_FDE_FUNC_START_PCREL, as an erratum to
SFrame Version 2, to indicate the encoding of the SFrame FDE function start
address field:
 @itemize @minus
  @item if set, @code{sfde_func_start_offset} field contains the offset in
bytes to the start PC of the associated function from the field itself.
  @item if unset, @code{sfde_func_start_offset} field contains the offset in
bytes to the start PC of the associated function from the start of the SFrame
section.
 @end itemize
@item
[Errata 1] Add a new ABI/arch identifier SFRAME_ABI_S390X_ENDIAN_BIG for the
s390 architecture (64-bit) s390x ABI.  Other s390x-specific backward compatible
changes including the following helper definitions have been incrementally
added to SFrame Version 2 only:
 @itemize @minus
  @item SFRAME_S390X_SP_VAL_OFFSET: SP value offset from CFA.
  @item SFRAME_V2_S390X_OFFSET_IS_REGNUM: Test whether FP/RA offset is an encoded
DWARF register number.
  @item SFRAME_V2_S390X_OFFSET_ENCODE_REGNUM: Encode a DWARF register number as an
FP/RA offset.
  @item SFRAME_V2_S390X_OFFSET_DECODE_REGNUM: Decode a DWARF register number from
an FP/RA offset.
  @item SFRAME_FRE_RA_OFFSET_INVALID: Invalid RA offset value (like
SFRAME_CFA_FIXED_RA_INVALID).  Used on s390x as padding offset to represent
FP without RA saved.
  @item SFRAME_S390X_CFA_OFFSET_ADJUSTMENT: CFA offset (from CFA base register)
adjustment value.  Used to enable use of 8-bit SFrame offsets on s390x.
  @item SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR: CFA offset alignment factor.
Used to scale down the CFA offset to improve the use of 8-bit SFrame offsets.
  @item SFRAME_V2_S390X_CFA_OFFSET_ENCODE: Encode CFA offset (i.e., apply
CFA offset adjustment and then scale down by CFA offset alignment factor).
  @item SFRAME_V2_S390X_CFA_OFFSET_DECODE: Decode CFA offset (i.e., scale up
by CFA offset alignment factor and then revert CFA offset adjustment).
 @end itemize
@item
[Errata 1] An ELF SFrame section has the type @code{SHT_GNU_SFRAME}.
@item
[Errata 2] An offset count of zero in the SFrame FRE info byte indicates that
the return address (RA) is undefined for the range of PCs covered by the SFrame
FRE.  A stack tracer may use this as indication that an outermost frame has
been reached and the stack trace is complete.
@end itemize

SFrame Version 1 is now obsolete and should not be used.

@node SFrame Section
@chapter SFrame Section
@cindex SFrame Section

The SFrame section consists of an SFrame header, starting with a preamble, and
two other sub-sections, namely the SFrame function descriptor entry (SFrame
FDE) sub-section, and the SFrame frame row entry (SFrame FRE) sub-section.

@menu
* SFrame Preamble::
* SFrame Header::
* SFrame Function Descriptor Entries::
* SFrame Frame Row Entries::
@end menu

@node SFrame Preamble
@section SFrame Preamble
@cindex SFrame preamble

The preamble is a 32-bit packed structure; the only part of the SFrame section
whose format cannot vary between versions.

@example
typedef struct sframe_preamble
@{
  uint16_t sfp_magic;
  uint8_t sfp_version;
  uint8_t sfp_flags;
@} ATTRIBUTE_PACKED sframe_preamble;
@end example

Every element of the SFrame preamble is naturally aligned.

All values are stored in the endianness of the target system for which the
SFrame section is intended.  Further details:

@multitable {Offset} {@code{uint16_t}} {@code{sfp_version}} {The magic number for SFrame section: 0xdee2.}
@headitem Offset @tab Type @tab Name @tab Description
@item 0x00
@tab @code{uint16_t}
@tab @code{sfp_magic}
@tab The magic number for SFrame section: 0xdee2.  Defined as a macro @code{SFRAME_MAGIC}.
@tindex SFRAME_MAGIC

@item 0x02
@tab @code{uint8_t}
@tab @code{sfp_version}
@tab The version number of this SFrame section.  @xref{SFrame Version}, for the
set of valid values.  Current version is
@code{SFRAME_VERSION_3}.

@item 0x03
@tab @code{uint8_t}
@tab @code{sfp_flags}
@tab Flags (section-wide) for this SFrame section.  @xref{SFrame Flags}, for the
set of valid values.
@end multitable

@menu
* SFrame Magic Number and Endianness::
* SFrame Version::
* SFrame Flags::
@end menu

@node SFrame Magic Number and Endianness
@subsection SFrame Magic Number and Endianness

@cindex endianness
@cindex SFrame magic number
SFrame sections are stored in the target endianness of the system that consumes
them.  A consumer library reading or writing SFrame sections should detect
foreign-endianness by inspecting the SFrame magic number in the
@code{sfp_magic} field in the SFrame header.  It may then provide means to
endian-flip the SFrame section as necessary.

@node SFrame Version
@subsection SFrame Version

The version of the SFrame format can be determined by inspecting
@code{sfp_version}.  The following versions are currently valid:

@tindex SFRAME_VERSION_1
@cindex SFrame versions
@multitable {SFRAME_VERSION_3} {Number} {Current version, under development.}
@headitem Version Name @tab Number @tab Description
@item @code{SFRAME_VERSION_1}
@tab 1 @tab First version, obsolete.
@item @code{SFRAME_VERSION_2}
@tab 2 @tab Second version.
@item @code{SFRAME_VERSION_3}
@tab 3 @tab Third version, under development.
@end multitable

This document describes @code{SFRAME_VERSION_3}.

@node SFrame Flags
@subsection SFrame Flags
@cindex SFrame Flags

The preamble contains bitflags in its @code{sfp_flags} field that
describe various section-wide properties.

The following flags are currently defined.

@multitable {@code{SFRAME_F_FRAME_POINTER}} {Version} {Value} {Function Descriptor Entries are sorted}
@headitem Flag @tab Version @tab Value @tab Meaning
@tindex SFRAME_F_FDE_SORTED
@item @code{SFRAME_F_FDE_SORTED} @tab All @tab 0x1 @tab Function Descriptor
Entries are sorted on PC.
@tindex SFRAME_F_FRAME_POINTER
@item @code{SFRAME_F_FRAME_POINTER} @tab 1-2 @tab 0x2
@tab All functions in the object file preserve frame pointer.
@tindex SFRAME_F_FDE_FUNC_START_PCREL
@item @code{SFRAME_F_FDE_FUNC_START_PCREL} @tab 2-3 @tab 0x4
@tab The @code{sfdi_func_start_offset} field in the SFrame FDE is an offset in
bytes to the function's start address, from the field itself.  If unset, the
@code{sfdi_func_start_offset} field in the SFrame FDE is an offset in bytes to
the function's start address, from the start of the SFrame section.
@end multitable

The purpose of @code{SFRAME_F_FRAME_POINTER} flag was to facilitate stack
tracers to reliably fallback on the frame pointer based stack tracing method,
if SFrame information is not present for some function in the SFrame section.

Further flags may be added in future.  Bits corresponding to the currently
undefined flags must be set to zero.

@node SFrame Header
@section SFrame Header
@cindex SFrame header

The SFrame header is the first part of an SFrame section.  It begins with the
SFrame preamble.  All parts of it other than the preamble
(@pxref{SFrame Preamble}) can vary between SFrame file versions.  It contains
metadata that apply to the section as a whole, and offsets to the various other
sub-sections defined in the format.  As with the rest of the SFrame section,
all values are stored in the endianness of the target system.

The two sub-sections tile the SFrame section: each section runs from the offset
given until the start of the next section.  An explicit length is given for the
last sub-section, the SFrame Frame Row Entry (SFrame FRE) sub-section.

@example
typedef struct sframe_header
@{
  sframe_preamble sfh_preamble;
  uint8_t sfh_abi_arch;
  int8_t sfh_cfa_fixed_fp_offset;
  int8_t sfh_cfa_fixed_ra_offset;
  uint8_t sfh_auxhdr_len;
  uint32_t sfh_num_fdes;
  uint32_t sfh_num_fres;
  uint32_t sfh_fre_len;
  uint32_t sfh_fdeoff;
  uint32_t sfh_freoff;
@} ATTRIBUTE_PACKED sframe_header;
@end example

Every element of the SFrame header is naturally aligned.

The sub-section offsets, namely @code{sfh_fdeoff} and @code{sfh_freoff}, in the
SFrame header are relative to the @emph{end} of the SFrame header; they are
each an offset in bytes into the SFrame section where the SFrame FDE
sub-section and the SFrame FRE sub-section respectively start.

The SFrame section contains @code{sfh_num_fdes} number of fixed-length array
elements in the SFrame FDE sub-section.  Each array element is of type SFrame
function descriptor entry; each providing a high-level function description for
the purpose of stack tracing.  More details in @ref{SFrame Function Descriptor
Entries}.

Next, the SFrame FRE sub-section, starting at offset @code{sfh_fre_off},
describes the stack trace information for each function.  For each function,
the SFrame FRE sub-section contains the SFrame FDE attribute data and
@code{sfh_num_fres} number of variable-length array elements.  Each array
element is of type SFrame frame row entry.  @xref{SFrame Frame Row Entries}.

SFrame header allows specifying explicitly the fixed offsets from CFA, if any,
from which FP or RA may be recovered.  For example, in AMD64, the stack offset
of the return address is @code{CFA - 8}.  Since these offsets are expected to
be in close vicinity to the CFA in most ABIs, @code{sfh_cfa_fixed_fp_offset}
and @code{sfh_cfa_fixed_ra_offset} are limited to signed 8-bit integers.

@cindex Provisions for future ABIs
The SFrame format has made some provisions for supporting more
ABIs/architectures in the future.  One of them is the concept of the auxiliary
SFrame header.  Bytes in the auxiliary SFrame header may be used to convey
further ABI-specific information.  The @code{sframe_header} structure provides
an unsigned 8-bit integral field to denote the size (in bytes) of an auxiliary
SFrame header.  The auxiliary SFrame header follows right after the
@code{sframe_header} structure.  As for the calculation of the sub-section
offsets, namely @code{sfh_fdeoff} and @code{sfh_freoff}, the @emph{end} of
SFrame header must be the end of the auxiliary SFrame header, if the latter is
present.

Putting it all together:

@multitable {Offset} {@code{uint32_t}} {@code{sfh_cfa_fixed_fp_offset}} {The number of SFrame FREs in the}
@headitem Offset @tab Type @tab Name @tab Description
@item 0x00
@tab @code{sframe_ @* preamble}
@tab @code{sfh_preamble}
@tab The SFrame preamble. @xref{SFrame Preamble}.

@item 0x04
@tab @code{uint8_t}
@tab @code{sfh_abi_arch}
@tab The ABI/arch identifier.  @xref{SFrame ABI/arch Identifier}.

@item 0x05
@tab @code{int8_t}
@tab @code{sfh_cfa_fixed_fp_offset}
@tab The CFA fixed FP offset, if any.

@item 0x06
@tab @code{int8_t}
@tab @code{sfh_cfa_fixed_ra_offset}
@tab The CFA fixed RA offset, if any.

@item 0x07
@tab @code{uint8_t}
@tab @code{sfh_auxhdr_len}
@tab Size in bytes of the auxiliary header that follows the
@code{sframe_header} structure.

@item 0x08
@tab @code{uint32_t}
@tab @code{sfh_num_fdes}
@tab The number of SFrame FDEs in the section.

@item 0x0c
@tab @code{uint32_t}
@tab @code{sfh_num_fres}
@tab The number of SFrame FREs in the section.

@item 0x10
@tab @code{uint32_t}
@tab @code{sfh_fre_len}
@tab The length in bytes of the SFrame FRE sub-section.

@item 0x14
@tab @code{uint32_t}
@tab @code{sfh_fdeoff}
@tab The offset in bytes to the SFrame FDE sub-section.

@item 0x18
@tab @code{uint32_t}
@tab @code{sfh_freoff}
@tab The offset in bytes to the SFrame FRE sub-section.

@end multitable

@menu
* SFrame ABI/arch Identifier::
@end menu

@node SFrame ABI/arch Identifier
@subsection SFrame ABI/arch Identifier
@cindex SFrame ABI/arch Identifier

SFrame header identifies the ABI/arch of the target system for which the
executable and hence, the stack trace information contained in the SFrame
section, is intended.  There are currently four identifiable ABI/arch values
in the format.

@multitable {SFRAME_ABI_AARCH64_ENDIAN_LITTLE} {Value} {@code{AArch64 little-endian}}
@headitem ABI/arch Identifier @tab Value @tab Description

@tindex SFRAME_ABI_AARCH64_ENDIAN_BIG
@item @code{SFRAME_ABI_AARCH64_ENDIAN_BIG}
@tab 1 @tab AArch64 big-endian

@tindex SFRAME_ABI_AARCH64_ENDIAN_LITTLE
@item @code{SFRAME_ABI_AARCH64_ENDIAN_LITTLE}
@tab 2 @tab AArch64 little-endian

@tindex SFRAME_ABI_AMD64_ENDIAN_LITTLE
@item @code{SFRAME_ABI_AMD64_ENDIAN_LITTLE}
@tab 3 @tab AMD64 little-endian

@tindex SFRAME_ABI_S390X_ENDIAN_BIG
@item @code{SFRAME_ABI_S390X_ENDIAN_BIG}
@tab 4 @tab s390x big-endian

@end multitable

The presence of an explicit identification of ABI/arch in SFrame may allow
stack trace generators to make certain ABI/arch-specific decisions.

@node SFrame Function Descriptor Entries
@section SFrame FDE
@cindex SFrame FDE

SFrame function descriptor entry is a conceptual entity which contains the
function-level metadata necessary for stack tracing through the function.  It
is composed of two physical entities: the SFrame function descriptor index
(SFrame FDE index) and the SFrame function descriptor attribute (SFrame FDE
attribute).  Both SFrame FDE index and SFrame FDE attribute are fixed-length
structures, albeit with different alignment guarantees.

@menu
* The SFrame FDE Index::
* The SFrame FDE Attribute::
* The SFrame FDE Info Bytes::
@end menu

@cindex The SFrame FDE Index
@node The SFrame FDE Index
@subsection The SFrame FDE Index

The SFrame FDE index entries are stored in a sub-section of their own, forming
a searchable index.  If the SFrame header flag @code{SFRAME_F_FDE_SORTED} is
set, then the entries are sorted by @code{sfdi_func_start_offset}, allowing for
efficient binary search.  Typically (as is the case with GNU ld) a linked
object or executable will have the @code{SFRAME_F_FDE_SORTED} set.  This makes
the job of a stack tracer easier as it may then employ a binary search scheme
to look for the stack trace information pertinent to a given PC.

@example
typedef struct sframe_func_desc_idx
@{
  int64_t sfdi_func_start_offset;
  uint32_t sfdi_func_size;
  uint32_t sfdi_func_start_fre_off;
@} ATTRIBUTE_PACKED sframe_func_desc_idx;
@end example

Each entry of the SFrame function descriptor index is naturally aligned.  The
following table describes each component of the SFrame FDE index entry:

@multitable {Offset} {@code{uint32_t}} {@code{sfdi_func_start_fre_off}} {Signed 32-bit integral field denoting the}
@headitem Offset @tab Type @tab Name @tab Description
@item 0x00
@tab @code{int64_t}
@tab @code{sfdi_func_start_offset}
@tab Signed 64-bit integral field specifying the offset to the start address of
the described function.  If the flag @code{SFRAME_F_FDE_FUNC_START_PCREL},
@xref{SFrame Flags}, in the SFrame header is set, the value encoded in the
@code{sfdi_func_start_offset} field is the offset in bytes to the function's
start address from the @code{sfdi_func_start_offset} field itself.  Otherwise,
it is the offset in bytes from the start of the SFrame section.

@item 0x08
@tab @code{uint32_t}
@tab @code{sfdi_func_size}
@tab Unsigned 32-bit integral field specifying the size of the function in
bytes.

@item 0x0c
@tab @code{uint32_t}
@tab @code{sfdi_func_start_fre_off}
@tab Unsigned 32-bit integral field specifying the offset to the start of the
function's stack trace data (SFrame FREs).  This offset is relative to the
@emph{beginning of the SFrame FRE sub-section}.

@end multitable

@cindex The SFrame FDE Attribute
@node The SFrame FDE Attribute
@subsection The SFrame FDE Attribute

The SFrame FDE attribute structure provides information about the SFrame FRE
entries that follow: their number and their encoding.  The SFrame FDE
attributes are stored at the beginning of each function's stack trace data
within the SFrame FRE sub-section.  Because these structures are interleaved
with variable-length FREs, their elements are not guaranteed to be at naturally
aligned boundaries.

@example
typedef struct sframe_func_desc_attr
@{
  uint16_t sfda_func_num_fres;
  uint8_t sfda_func_info;
  uint8_t sfda_func_info2;
  uint8_t sfda_func_rep_size;
@} ATTRIBUTE_PACKED sframe_func_desc_attr;
@end example

Following table describes each component of the SFrame FDE attribute:

@multitable {Offset} {@code{uint16_t}} {@code{sfda_func_rep_size}} {Unsigned 16-bit integral field specifying the}
@headitem Offset @tab Type @tab Name @tab Description
@item 0x00
@tab @code{uint16_t}
@tab @code{sfda_func_num_fres}
@tab Unsigned 16-bit integral field specifying the total number of SFrame FREs
used for the function.

@item 0x02
@tab @code{uint8_t}
@tab @code{sfda_func_info}
@tab Unsigned 8-bit integral field specifying the SFrame FDE info byte.

@item 0x03
@tab @code{uint8_t}
@tab @code{sfda_func_info2}
@tab Additional unsigned 8-bit integral field specifying the SFrame FDE info byte.

@item 0x04
@tab @code{uint8_t}
@tab @code{sfda_func_rep_size}
@tab Unsigned 8-bit integral field specifying the size of the repetitive code
block for which an SFrame FDE of type @code{SFRAME_FDE_PCTYPE_MASK} is used.  For
example, in AMD64, the size of a pltN entry is 16 bytes.

@end multitable

@code{sfda_func_info} and @code{sfda_func_info2} are the SFrame FDE
@strong{Info Bytes}, containing information like the FRE type and their
encoding, and the FDE type for the function.  @xref{The SFrame FDE Info Bytes}.

@cindex Provisions for future ABIs
The SFrame FDE attribute has some currently unused bits in the SFrame FDE info
bytes, that may be used for the purpose of extending the SFrame file format
specification for future ABIs.  @xref{The SFrame FDE Types} subsection.

@cindex The SFrame FDE Info Bytes
@node The SFrame FDE Info Bytes
@subsection The SFrame FDE Info Bytes

The SFrame FDE Attribute contains two distinct bytes, @code{sfda_func_info} and
@code{sfda_func_info2}.  Together these are referred to as the SFrame FDE info
bytes.  These bytes contain vital information necessary to:

@itemize @minus
 @item read and interpret SFrame FRE data, e.g., the number and size of each
SFrame FRE offset,
 @item PC Type for SFrame FDE,
 @item type of SFrame FDE,
 @item size of repeat block, if PC Type is @code{SFRAME_FDE_PCTYPE_MASK}.
@end itemize

The first info byte @code{sfda_func_info} is a bitfield split into four parts.
From MSB to LSB:

@multitable {Bit offset} {@code{fde_pctype}} {Specify which key is used for signing the return addresses}
@headitem Bit offset @tab Name @tab Description

@item 7
@tab @code{signal_p}
@tab Signal frame.

@item 6
@tab @code{unused}
@tab Unused bit.

@item 5
@tab @code{pauth_key}
@tab (For AArch64) Specify which key is used for signing the return addresses
in the SFrame FDE.  Two possible values: @*
@code{SFRAME_AARCH64_PAUTH_KEY_A} (0), or @*
@code{SFRAME_AARCH64_PAUTH_KEY_B} (1). @*
Unsed in AMD64, s390x

@item 4
@tab @code{fde_pctype}
@tab Specify the SFrame FDE PC Type.  Two possible values: @*
@code{SFRAME_FDE_PCTYPE_MASK} (1), or @*
@code{SFRAME_FDE_PCTYPE_INC} (0). @*
@xref{The SFrame FDE PC Types}.

@item 0--3
@tab @code{fre_type}
@tab Choice of three SFrame FRE types. @xref{The SFrame FRE Types}.
@end multitable

The second info byte @code{sfda_func_info2} is a bitfield split into two parts.
From MSB to LSB:

@multitable {Bit offset} {@code{fde_type}} {Specify which key is used for signing the return addresses}
@headitem Bit offset @tab Name @tab Description
@item 7--5
@tab @code{unused}
@tab Unused bits.

@item 4--0
@tab @code{fde_type}
@tab Specify the SFrame FDE type.  Two possible values: @*
@code{SFRAME_FDE_TYPE_DEFAULT} (0), or @*
@code{SFRAME_FDE_TYPE_FLEX} (1). @*
@xref{The SFrame FDE Types}.

@end multitable

@menu
* The SFrame FDE PC Types::
* The SFrame FDE Types::
* The SFrame FRE Types::
@end menu

@node The SFrame FDE PC Types
@subsubsection The SFrame FDE PC Types
@tindex SFRAME_V3_FDE_PCTYPE_MASK
@tindex SFRAME_V3_FDE_PCTYPE_INC

The SFrame format defines two types of FDE PC types.  The choice of which SFrame
FDE PC type to use is made based on the instruction patterns in the relevant
program stub.

An FDE of PC type @code{SFRAME_V3_FDE_PCTYPE_INC} contains FREs whose PCs are
to be interpreted as the address of a single instruction, measured in bytes and
relative to the beginning of the function.

In contrast, a FDE of PC type @code{SFRAME_V3_FDE_PCTYPE_MASK} contains FREs
whose PCs are to be interpreted as masks that identify several instructions.
This is useful for cases where a small pattern of instructions in a program
stub is used repeteadly for a specific functionality, like PLT entries and
trampolines.

@multitable {@code{SFRAME_V3_FDE_PCTYPE_MASK}} {Value} {Unwinders perform a Unwinders perform a fo}
@headitem Name of SFrame FDE PC Type @tab Value @tab Description

@item @code{SFRAME_V3_FDE_PCTYPE_INC}
@tab 0 @tab Stacktracers perform a @*
(PC >= FRE_START_ADDR) to look up a matching FRE.

@item @code{SFRAME_V3_FDE_PCTYPE_MASK}
@tab 1 @tab  Stacktracers perform a @*
(PC % REP_BLOCK_SIZE @*
 >= FRE_START_ADDR)
to look up a matching FRE.  REP_BLOCK_SIZE is the size in bytes of the
repeating block of program instructions and is encoded via
@code{sfde_func_rep_size} in the SFrame FDE.

@end multitable

@node The SFrame FDE Types
@subsubsection The SFrame FDE Types
@cindex The SFrame FDE Types
@tindex SFRAME_FDE_TYPE_DEFAULT
@tindex SFRAME_FDE_TYPE_FLEX

The SFrame format defines two types of Function Descriptor Entries (FDEs) to
encode stack trace information.  The choice of FDE type determines how the data
in the variable-length Frame Row Entries (FREs) is interpreted.  The FDE type
is encoded in the lower 5 bits of the @code{sfda_func_info2} field in the
SFrame FDE attribute.

@multitable {@code{SFRAME_FDE_TYPE_DEFAULT}} {Value} {CFA is recovered using the Stack Pointer (SP) use}
@headitem Name @tab Value @tab Description
@tindex SFRAME_FDE_TYPE_DEFAULT
@item @code{SFRAME_FDE_TYPE_DEFAULT}
@tab 0
@tab The default FDE type. @*
CFA is recovered using the Stack Pointer (SP) or Frame Pointer (FP) plus a
signed offset.  Return Address (RA) and Frame Pointer (FP) are recovered using
the CFA plus a signed offset (or a fixed register for specific
architectures like s390x).

The variable-length array of bytes trailing each SFrame FRE are interpreted
according to the ABI/arch-specific rules for the target architecture.  More
details in @ref{Default FDE Type Interpretation}.

@tindex SFRAME_FDE_TYPE_FLEX
@item @code{SFRAME_FDE_TYPE_FLEX}
@tab 1
@tab The flexible FDE type. @*
Used for complex cases such as stack realignment (DRAP), non-standard CFA base
registers, or when RA/FP recovery requires dereferencing or non-CFA base
registers.

The variable-length array of bytes may be interpreted as pairs of Control Data
and Offset Data (or Padding Data), allowing for complex recovery rules (e.g.,
DRAP on AMD64, Stack Realignment).  More details in @ref{Flexible FDE Type
Interpretation}.
@end multitable

@cindex Provisions for future ABIs
Currently, five bits are reserved in the @code{sfda_func_info2} for indicating
SFrame FDE types.  In future, other ABIs/architectures may add even
arch-specific FDE types.  Each distinct FDE type may define a different layout,
encoding, and interpretation of the variable-length data words trailing each
SFrame FRE.


@node The SFrame FRE Types
@subsubsection The SFrame FRE Types
@cindex The SFrame FRE Types

A real world application can have functions of size big and small.  SFrame
format defines three types of SFrame FRE entries to efficiently encode the
stack trace information for such a variety of function sizes.  These
representations vary in the number of bits needed to encode the start address
offset in the SFrame FRE.

The following constants are defined and used to identify the SFrame FRE types:

@multitable {SFRAME_FRE_TYPE_ADDR1} {@code{Value}} {The start address offset (in bytes) of the}
@headitem Name @tab Value @tab Description

@tindex SFRAME_FRE_TYPE_ADDR1
@item @code{SFRAME_FRE_TYPE_ADDR1}
@tab 0
@tab The start address offset (in bytes) of the SFrame FRE is an unsigned
8-bit value.

@tindex SFRAME_FRE_TYPE_ADDR2
@item @code{SFRAME_FRE_TYPE_ADDR2}
@tab 1
@tab The start address offset (in bytes) of the SFrame FRE is an unsigned
16-bit value.

@tindex SFRAME_FRE_TYPE_ADDR4
@item @code{SFRAME_FRE_TYPE_ADDR4}
@tab 2
@tab The start address offset (in bytes) of the SFrame FRE is an unsigned
32-bit value.
@end multitable

A single function must use the same type of SFrame FRE throughout.  The
identifier to reflect the chosen SFrame FRE type is stored in the
@code{fre_type} bits in the SFrame FDE info byte,
@xref{The SFrame FDE Info Bytes}.

@node SFrame Frame Row Entries
@section SFrame FRE
@cindex SFrame FRE

The SFrame frame row entry sub-section contains the core of the stack trace
information.  An SFrame frame row entry (FRE) is a self-sufficient record
containing SFrame stack trace information for a range of contiguous
(instruction) addresses, starting at the specified offset from the start of the
function.

Each SFrame FRE encodes the information to recover the CFA, FP and RA (as
specified by the ABI or the FDE type) for the respective instruction addresses.
To encode this information, each SFrame FRE is followed by S*N bytes, where:

@itemize @minus
@item
@code{S} is the size of each data word in the variable-length array of data
words trailing the SFrame FRE, and
@item
@code{N} is the number of data words trailing the SFrame FRE.
@end itemize

@strong{NB:} The term `data word' is used throughout this specification in a
colloquial sense to denote a discrete unit of information within an SFrame
Frame Row Entry (FRE). It is intended to describe the semantic role of the data
rather than its physical size. Consequently, `data word' should not be
understood to correlate with the architectural machine word size or any
specific hardware data width; the actual size of a data word in the SFrame
format is variable and is defined in the SFrame FRE info byte.

The entities @code{S}, @code{N} are encoded in the SFrame FRE info byte, via
the @code{fre_dataword_size} and the @code{fre_dataword_count} respectively.
More information about the precise encoding and range of values for @code{S}
and @code{N} is provided later in the @ref{The SFrame FRE Info Word}.

@cindex Provisions for future ABIs
It is important to underline here that although the canonical interpretation of
these data words is as stack offsets (to recover CFA, FP and RA) for default
FDE type, these bytes @emph{may} be used by future ABIs/architectures to convey
other information on a per SFrame FRE basis.

In summary, SFrame file format, by design, supports a variable length array of
bytes at the tail end of each SFrame FRE.  To keep the SFrame file format
specification flexible yet extensible, the interpretation of these bytes is
specific to ABI/arch or FDE type.  More details about the precise
interpretation are covered in the section @ref{Interpretation of SFrame FREs}.

Next, the definitions of the three SFrame FRE types are as follows:

@example
typedef struct sframe_frame_row_entry_addr1
@{
  uint8_t sfre_start_address;
  sframe_fre_info sfre_info;
@} ATTRIBUTE_PACKED sframe_frame_row_entry_addr1;
@end example

@example
typedef struct sframe_frame_row_entry_addr2
@{
  uint16_t sfre_start_address;
  sframe_fre_info sfre_info;
@} ATTRIBUTE_PACKED sframe_frame_row_entry_addr2;
@end example

@example
typedef struct sframe_frame_row_entry_addr4
@{
  uint32_t sfre_start_address;
  sframe_fre_info sfre_info;
@} ATTRIBUTE_PACKED sframe_frame_row_entry_addr4;
@end example

For ensuring compactness, SFrame frame row entries are stored unaligned on
disk.  Appropriate mechanisms need to be employed, as necessary, by the
serializing and deserializing entities, if unaligned accesses need to be
avoided.

@code{sfre_start_address} is an unsigned 8-bit/16-bit/32-bit integral field
denoting the start address of a range of program counters, for which the
SFrame FRE applies.  The value encoded in the @code{sfre_start_address} field
is the offset in bytes of the range's start address, from the start address
of the function.

Further SFrame FRE types may be added in future.

@menu
* The SFrame FRE Info Word::
@end menu

@cindex The SFrame FRE Info Word
@node The SFrame FRE Info Word
@subsection The SFrame FRE Info Word

The SFrame FRE info byte is a bitfield split into four parts.  From MSB to LSB:

@multitable {Bit offset} {@code{fre_cfa_base_reg_id}} {Being a 4-bit sized field, a max value of 15 is allowed.}
@headitem Bit offset @tab Name @tab Description
@item 7
@tab @code{fre_mangled_ra_p}
@tab Indicate whether the return address is mangled with any authorization bits (signed RA).

@item 5-6
@tab @code{fre_dataword_size}
@tab Size of data word in bytes.  Valid values are: @*
@code{SFRAME_FRE_DATAWORD_1B}, @*
@code{SFRAME_FRE_DATAWORD_2B}, and @*
@code{SFRAME_FRE_DATAWORD_4B}.

@item 1-4
@tab @code{fre_dataword_count}
@tab Being a 4-bit sized field, a max value of 15 is allowed.  Typically, a
value of up to 3 is sufficient for most ABIs to track all three of CFA, FP and
RA.  A value of zero indicates that the return address (RA) is undefined.  A
stack tracer may use this as indication that an outermost frame has been
reached and the stack trace is complete.

@item 0
@tab @code{fre_cfa_base_reg_id}
@tab Distinguish between SP or FP based CFA recovery.

@end multitable

@multitable {@code{SFRAME_FRE_DATAWORD_4B}} {@code{Value}} {All stack offsets following the fixed-length}
@headitem Name @tab Value @tab Description

@tindex SFRAME_FRE_DATAWORD_1B
@item @code{SFRAME_FRE_DATAWORD_1B}
@tab 0
@tab All data words following the fixed-length FRE structure are 1 byte
long.

@tindex SFRAME_FRE_DATAWORD_2B
@item @code{SFRAME_FRE_DATAWORD_2B}
@tab 1
@tab All data words following the fixed-length FRE structure are 2 bytes
long.

@tindex SFRAME_FRE_DATAWORD_4B
@item @code{SFRAME_FRE_DATAWORD_4B}
@tab 2
@tab All data words following the fixed-length FRE structure are 4 bytes
long.

@end multitable

@node Interpretation of SFrame FREs
@chapter Interpretation of SFrame FREs
@cindex Interpretation of SFrame FREs

Each SFrame Frame Row Entry (FRE) provides information about a PC range within
some function, encoded using a variable number of bytes (@pxref{SFrame Frame
Row Entries}).  The interpretation of these bytes depends on the FDE type
used to represent stack tracing information for the function.

@menu
* Default FDE Type Interpretation::
* Flexible FDE Type Interpretation::
@end menu

@node Default FDE Type Interpretation
@section Default FDE Type Interpretation
@cindex SFRAME_FDE_TYPE_DEFAULT

If the FDE type is @code{SFRAME_FDE_TYPE_DEFAULT}, the interpretation of the
FRE bytes is ABI/arch-specific.  Typically, these bytes are interpreted as a
sequence of (signed integer) stack offsets.

The following sections describe the specific interpretation rules for currently
supported architectures.

@menu
* AMD64::
* AArch64::
* s390x::
@end menu

@node AMD64
@subsection AMD64

Irrespective of the ABI, the first stack offset is always used to locate the
CFA, by interpreting it as: CFA = @code{BASE_REG} + offset1.  The
identification of the @code{BASE_REG} is done by using the
@code{fre_cfa_base_reg_id} field in the SFrame FRE info byte.

In AMD64, the return address (RA) is always saved on stack when a function
call is executed.  Further, AMD64 ABI mandates that the RA be saved at a
@code{fixed offset} from the CFA when entering a new function.  This means
that the RA does not need to be tracked per SFrame FRE.  The fixed offset is
encoded in the SFrame file format in the field @code{sfh_cfa_fixed_ra_offset}
in the SFrame header.  @xref{SFrame Header}.

Hence, the second stack offset (in the SFrame FRE), when present, will be used
to locate the FP, by interpreting it as: FP = CFA + offset2.

Hence, in summary:

@multitable {Offset ID} {Interpretation in AMD64 in AMD64}
@headitem Offset ID @tab Interpretation in AMD64
@item 1 @tab CFA = @code{BASE_REG} + offset1
@item 2 @tab FP = CFA + offset2
@end multitable

@node AArch64
@subsection AArch64

Irrespective of the ABI, the first stack offset is always used to locate the
CFA, by interpreting it as: CFA = @code{BASE_REG} + offset1.  The
identification of the @code{BASE_REG} is done by using the
@code{fre_cfa_base_reg_id} field in the SFrame FRE info byte.

In AArch64, the AAPCS64 standard specifies that the Frame Record saves both FP
and LR (a.k.a the RA).  However, the standard does not mandate the precise
location in the function where the frame record is created, if at all.  Hence
the need to track RA in the SFrame stack trace format.  As RA is being tracked
in this ABI, the second stack offset is always used to locate the RA, by
interpreting it as: RA = CFA + offset2.  The third stack offset will be used to
locate the FP, by interpreting it as: FP = CFA + offset3.

Given the nature of things, the number of stack offsets seen on AArch64 per
SFrame FRE is either 1 or 3.

Hence, in summary:

@multitable {Offset ID} {Interpretation in AArch64 in X}
@headitem Offset ID @tab Interpretation in AArch64
@item 1 @tab CFA = @code{BASE_REG} + offset1
@item 2 @tab RA = CFA + offset2
@item 3 @tab FP = CFA + offset3
@end multitable

@node s390x
@subsection s390x

A stack tracer implementation must initialize the SP to the designated SP
register value, the FP to the preferred FP register value, and the RA to the
designated RA register value in the topmost stack frame of the callchain.  This
is required, as either the SP or FP is used as CFA base register and as the FP
and/or RA are not necessarily saved on the stack.  For RA this may only be the
case in the topmost stack frame of the callchain.  For FP this may be the case
in any stack frame.

Irrespective of the ABI, the first stack offset is always used to locate the
CFA.  On s390x the value of the offset is stored adjusted by the s390x-specific
@code{SFRAME_S390X_CFA_OFFSET_ADJUSTMENT} and scaled down by the s390x-specific
@code{SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR}, to enable and improve the use
of signed 8-bit offsets on s390x.
s390x-specific helpers @code{SFRAME_V2_S390X_CFA_OFFSET_ENCODE} and
@code{SFRAME_V2_S390X_CFA_OFFSET_DECODE} are provided to perform or undo
the adjustment and scaling.  The CFA offset can therefore be interpreted as:
CFA = @code{BASE_REG} + offset1 - @code{SFRAME_S390X_CFA_OFFSET_ADJUSTMENT}
or
CFA = @code{BASE_REG}
    + (offset1 * @code{SFRAME_S390X_CFA_OFFSET_ALIGNMENT_FACTOR})
    - @code{SFRAME_S390X_CFA_OFFSET_ADJUSTMENT}.
The identification of the @code{BASE_REG} is done by using the
@code{fre_cfa_base_reg_id} field in the SFrame FRE info byte.

The (64-bit) s390x ELF ABI does not mandate the precise location in a function
where the return address (RA) and frame pointer (FP) are saved, if at all.
Hence the need to track RA in the SFrame stack trace format.  As RA is being
tracked in this ABI, the second stack offset is always used to locate the RA
stack slot, by interpreting it as: RA = CFA + offset2, unless the offset has a
value of @code{SFRAME_FRE_RA_OFFSET_INVALID}.  RA remains unchanged, if the
offset is not available or has a value of @code{SFRAME_FRE_RA_OFFSET_INVALID}.
Stack tracers are recommended to validate that the "unchanged RA" pattern, when
present, is seen only for the topmost stack frame.  The third stack offset is
used to locate the FP stack slot, by interpreting it as: FP = CFA + offset3.
FP remains unchanged, if the offset is not available.

In leaf functions the RA and FP may be saved in other registers, such as
floating-point registers (FPRs), instead of being saved on the stack.  To
represent this in the SFrame stack trace format, SFrame FDE of type
@code{SFRAME_FDE_TYPE_FLEX} may be used.

Given the nature of things, for default type FDEs, the number of stack offsets
seen on s390x per SFrame FRE is either 1, 2, or 3.

Hence, in summary:

@multitable @columnfractions .15 .85
@headitem Offset ID @tab Interpretation in s390x
@item 1 @tab CFA = @code{BASE_REG} + offset1
@item 2 @tab RA stack slot = CFA + offset2
           @*RA not saved if (offset2 == @code{SFRAME_FRE_RA_OFFSET_INVALID})
@item 3 @tab FP stack slot = CFA + offset3
@end multitable

The s390x ELF ABI defines the CFA as stack pointer (SP) at call site +160.  The
SP can therefore be obtained using the SP value offset from CFA
@code{SFRAME_S390X_SP_VAL_OFFSET} of -160 as follows:
SP = CFA + @code{SFRAME_S390X_SP_VAL_OFFSET}

Future ABIs must specify the algorithm for identifying the appropriate SFrame
FRE stack offsets in this chapter.  This should inevitably include the
blueprint for interpreting the variable number of bytes at the tail end of the
SFrame FRE for the specific ABI/arch.


@node Flexible FDE Type Interpretation
@section Flexible FDE Type Interpretation
@cindex SFRAME_FDE_TYPE_FLEX

Flexible FDEs (@code{SFRAME_FDE_TYPE_FLEX}) are used in cases where the most
common default recovery rules implied by @code{SFRAME_FDE_TYPE_DEFAULT} are
insufficient.  Common use cases include:
@itemize @bullet
@item
DRAP (Dynamically Realigned Argument Pointer): Where the CFA is based on a
register other than SP or FP, or requires dereferencing.
@item
Stack Realignment: Where strict alignment requirements (e.g., AVX512)
force dynamic stack adjustments.
@item
Register-based RA/FP Locations: Where the Return Address or Frame Pointer is
transiently saved in a general-purpose register and/or requires a dereference
rule.
@end itemize

For flexible FDE types, the variable-length bytes trailing an SFrame FRE can be
interpreted as one of the following:

@enumerate
@item Control Data: Encodes the base register number, a dereference flag, and a
register-mode flag.  A value of 0 is reserved as the @emph{padding data word}.
@item Offset Data: Encodes the signed offset to be added to the base.
@end enumerate

For each tracked entity (CFA, RA, FP), the SFrame FRE carries a pair of data
words to specify the respective recovery rule.  The pair of data words appear
in the order: CFA, RA, FP.  These data words obey the @code{fre_dataword_size}
defined in the FRE info byte (i.e., they are 1, 2, or 4 bytes wide).

Given the nature of things, since CFA is always tracked, the first two data
words pertain to CFA recovery.  If RA recovery rule is unspecified (because the
RA can be recovered from its default location), a single padding data word is
used instead of the pair of Control data word and Offset data word if FP
recovery rule is to be specified using the subsequent data words.

Following is the order of information for specifying the recovery rule for a
tracked entity in a flexible FDE.

@subsubheading Encoding of Data Word 1 (Control Data)

The first data word of the pair is an unsigned integer of size
@code{fre_dataword_size}.  It is used as a bitfield that describes
register/control data for the tracked entity.  From LSB to MSB:

@multitable {Bit Offset} {@code{deref_p}} {If 1, the base is a DWARF register (encoded in bits 3+}
@headitem Bit Offset @tab Name @tab Description
@item 0
@tab @code{reg_p}
@tab Register-based Location Rule @*
If 1, the base is a DWARF register (encoded in bits 3+).
If 0, the base is the CFA (used for RA/FP recovery).
@item 1
@tab @code{deref_p}
@tab Dereference Flag @*
If 1, the location of the value is the address (@code{Base + Offset}), i.e.,
value = @code{*(Base + Offset)}. @*
If 0, the value is @code{Base + Offset}.
@item 2
@tab @code{unused}
@tab Unused bit.
@item 3+
@tab @code{regnum}
@tab The DWARF register number used as the base.  Effective only if
@code{reg_p} is 1.
@end multitable

A value of 0 (i.e., regnum = 0, deref_p = 0, reg_p = 0) in the Control Data
Word is used to indicate that no further data words follow for the tracked
entity.  This is to convey an absence of recovery rule for the respective
tracked entity (which means that fixed offsets @code{sfh_cfa_fixed_fp_offset}
or @code{sfh_cfa_fixed_ra_offset} apply if used for the ABI/arch).  Note that,
using a value of 0 as padding data word, does mean that currently, e.g., for
RA, the rule RA = CFA + 0 cannot be encoded.  NB: RA = CFA + 0 is distinct from
RA = *(CFA + 0).  The former should not be needed for any ABI, and the latter
is representable (regnum = 0, deref_p = 1, reg_p = 0).

@subsubheading Encoding of Data Word 2 (Offset Data)

The second data word of the pair is a signed integer of width
@code{fre_dataword_size}.  It is used as a offset for the respective tracked
entity (CFA, FP or RA).

@subsubheading Recovery Rules

The value of the tracked entity (CFA, RA, or FP) is calculated using the
following logic:

@example
    Base   = (reg_p == 1) ? Register[regnum] : CFA;
    Addr   = Base + Offset2;
    Value  = (deref_p == 1) ? *Addr : Addr;
@end example

@noindent
Examples:

@itemize @bullet
@item
CFA = *(RBP - 8): (Typical DRAP pattern on AMD64)
@* Data Word 1: @code{(RBP << 3) | (1 << 1) | 1} (Reg RBP, deref_p=True, reg_p=True)
@* Data Word 2: @code{-8}

@item
FP = *(RBP + 0):
@* Data Word 1: @code{(RBP << 3) | (1 << 1) | 1} (Reg RBP, deref_p=True, reg_p=True)
@* Data Word 2: @code{0}

@item
RA = *(CFA - 8): (Standard RA recovery on AMD64)
@* Data Word 1: @code{(0 << 3 | (1 << 1) | 0)} (reg_p=False, implies Base=CFA,
deref_p=True by implication of standard stack save)
@* Data Word 2: @code{-8}
@end itemize

If the FDE type is @code{SFRAME_FDE_TYPE_FLEX}, the FRE bytes are interpreted
using a universal encoding scheme designed to handle complex recovery rules
(such as DRAP or non-standard RA locations).

@node Generating Stack Traces using SFrame
@appendix Generating Stack Traces using SFrame

Using some C-like pseudocode, this section highlights how SFrame provides a
simple, fast and low-overhead mechanism to generate stack traces.  Needless to
say that for generating accurate and useful stack traces, several other aspects
will need attention: finding and decoding bits of SFrame section(s) in the
program binary, symbolization of addresses, to name a few.

In the current context, a @code{frame} is the abstract construct that
encapsulates the following information:
@itemize @minus
@item
program counter (PC),
@item
stack pointer (SP), and
@item
frame pointer (FP)
@end itemize

With that said, establishing the first @code{frame} should be trivial:

@example
    // frame 0
    frame->pc = current_IP;
    frame->sp = get_reg_value (REG_SP);
    frame->fp = get_reg_value (REG_FP);
@end example

where @code{REG_SP} and @code{REG_FP} are are ABI-designated stack pointer and
frame pointer registers respectively.

Next, given frame N, generating stack trace needs us to get frame N+1.  This
can be done as follows:

@example
     // Get the PC, SP, and FP for frame N.
     pc = frame->pc;
     sp = frame->sp;
     fp = frame->fp;
     // Populate frame N+1.
     int err = get_next_frame (&next_frame, pc, sp, fp);
@end example


where given the values of the program counter, stack pointer and frame pointer
from frame N, @code{get_next_frame} populates the provided @code{next_frame}
object and returns the error code, if any.

In the following pseudocode for @code{get_next_frame}, the @code{sframe_*}
functions fetch information from the SFrame section.  Note that the stack tracer
must retrieve the FDE type to decide how to interpret the FRE data words.

@example
    fre = sframe_find_fre (pc, &fde_type);
    if (fre && fde_type == SFRAME_FDE_TYPE_DEFAULT)
        // Whether the base register for CFA tracking is REG_FP.
        base_reg_val = sframe_fre_base_reg_fp_p (fre) ? fp : sp;
        // Get the CFA stack offset from the FRE.
        cfa_offset = sframe_fre_get_cfa_offset (fre);
        // Get the fixed RA offset or FRE stack offset as applicable.
        ra_offset = sframe_fre_get_ra_offset (fre);
        // Get the fixed FP offset or FRE stack offset as applicable.
        fp_offset = sframe_fre_get_fp_offset (fre);

        cfa = base_reg_val + cfa_offset;
        next_frame->sp = cfa [+ SFRAME_S390X_SP_VAL_OFFSET on s390x];

        ra_stack_loc = cfa + ra_offset;
        // Get the address stored in the stack location.
        next_frame->pc = read_value (ra_stack_loc);

        if (fp_offset is VALID)
            fp_stack_loc = cfa + fp_offset;
            // Get the value stored in the stack location.
            next_frame->fp = read_value (fp_stack_loc);
        else
            // Continue to use the value of fp as it has not
            // been clobbered by the current frame yet.
            next_frame->fp = fp;
@end example

For SFrame FDE of type @code{SFRAME_FDE_TYPE_FLEX}, read the set of data words
and apply the recovery rules accordingly.

@example
    if (fre && fde_type == SFRAME_FDE_TYPE_FLEX)
        // Get the base register, offset, and deref_p for CFA tracking.
        // The first FRE offset (index 0) is the CFA Control Data.
        cfa_reg_data = sframe_fre_get_offset (fre, 0);
        cfa_offset = sframe_fre_get_offset (fre, 1);

        // Get the RA reg, offset, and deref_p.
        // The third FRE data word (index 2) is the RA Control Data.
        ra_reg_data = sframe_fre_get_udata (fre, 2);
        if (ra_reg_data != SFRAME_FRE_RA_OFFSET_INVALID)
                ra_offset = sframe_fre_get_offset (fre, 3);
                fp_tracking_p = fre.num_offsets > 3;
                fp_data_index = 3;
        else
                fp_tracking_p = fre.num_offsets > 4;
                fp_data_index = 4;

        // Get the FP reg, offset, and deref_p (if present).
        if (fp_tracking_p)
             fp_reg_data = sframe_fre_get_udata (fre, fp_data_index);
             fp_offset = sframe_fre_get_fp_offset (fre);

        // Safety check for topmost frames:
        // If recovery requires non-standard registers (not SP/FP),
        // it is only valid if we are at the top of the stack
        // (where those registers haven't been clobbered).
        cfa_base_reg = SFRAME_V3_FLEX_FDE_OFFSET_REG_NUM (cfa_reg_data);
        if (!topmost_frame_p && (cfa_base_reg != REG_FP
                                 && cfa_base_reg != REG_SP))
             return ERR_SFRAME_UNSAFE_UNWIND;

        // Apply rules to recover CFA and RA
        cfa = sframe_apply_rule (cfa_reg_data, cfa_offset, cfa, 1);
        ra = sframe_apply_rule (ra_reg_data, ra_offset, cfa, 0);

        if (fp_tracking_p)
            next_frame->fp
               = sframe_apply_rule (fp_reg_data, fp_offset, cfa, 0);
        else
            next_frame->fp = fp;

        next_frame->sp = cfa;
        next_frame->pc = ra;
    else
        ret = ERR_NO_SFRAME_FRE;
@end example

The @code{sframe_apply_rule} helper function abstracts the logic of
interpreting the Control Data and Offset Data pair for flexible FDEs:

@example
    // Apply SFrame V3 Flex FDE recovery rule.
    // reg_data: The Control Data (Data word 1)
                 containing reg_p, deref_p, regnum.
    // offset:   The Offset (Data word 2).
    // cfa:      The current CFA value (used as base if reg_p is 0).
    // cfa_p:    Bool indicating if we are currently recovering the
                 CFA itself.

    sframe_apply_rule (reg_data, offset, cfa, cfa_p)
        reg_p = SFRAME_V3_FLEX_FDE_OFFSET_REG_P (reg_data);

        // Determine Base Address:
        // If reg_p is set, read from the specific DWARF register.
        // If reg_p is clear, use the CFA (unless we are recovering the
        // CFA itself, in which case reg_p MUST be set).
        if (reg_p)
             reg_num = SFRAME_V3_FLEX_FDE_OFFSET_REG_NUM (reg_data);
             base_loc = get_reg_value (reg_num);
        else
             base_loc = cfa;

        // CFA recovery must always specify a base register.
        assert (!cfa_p || reg_p);

        // Add the displacement
        loc = base_loc + offset;

        // Dereference if required
        deref_p = SFRAME_V3_FLEX_FDE_OFFSET_REG_DEREF_P (reg_data);
        value = deref_p ? read_value (loc) : loc;

        return value;
@end example

@node Index
@unnumbered Index

@syncodeindex tp cp
@printindex cp

@bye