s390: Prevent GOT access rewrite for misaligned symbols
Dereferences of GOT slots with lgrl or lg for global symbols are
rewritten to larl to get get rid of the extra memory access. However
this is invalid for:
- symbols marked for absolute addressing
- symbols at odd addresses (larl can handle only even addresses)
Commit e6213e09ed0e ("S/390: Prevent GOT access rewrite for certain
symbols") added checks for the above. But instead of checking the
address of a symbol for being halfword aligned, it tries to deduce
this from whether the symbol value and section the symbol is defined
in are halfword aligned. The way it is done has two issues:
1. The use of bfd_section_from_elf_index to obtain the section the
symbol is defined in may not return the one that remains in the
output. For instance for COMDAT sections getting deduplicated
the section retrieved using bfd_section_from_elf_index may not be
the same as h->root.u.def.section. If COMDAT sections of same
group signature have different alignment properties the wrong
one may be checked. This may then lead to an erroneous rewrite
of lgrl %rX, sym@GOTENT to larl %rX, sym, although the symbol in
the remaining section is not properly aligned, triggering an
"relocation for misaligned symbol" error at link-time.
This may for instance occur when mixing C++ modules compiled with
GCC and Clang, as GCC emits a 2-byte alignment and Clang a 1-byte
alignment for COMDAT sections containing type information:
$ cat sample.cpp
#include <typeinfo>
struct A {};
const std::type_info &q() { return typeid(A); }
$ g++ -c sample.cpp -o sample_gcc.o
$ clang++ -c sample.cpp -o sample_clang.o
$ readelf -WS sample_gcc.o sample_clang.o
Produces (reformatted and reduced):
File Name Off Size ES Flg Lk Inf Al
sample_gcc.o .rodata._ZTS1A 000080 000004 00 AG 0 0 2
sample_clang.o .rodata._ZTS1A 000058 000003 00 AG 0 0 1
2. The symbol may end up at an even address, if both the symbol value
and the section defining the symbol are 1-byte aligned. While this
does not trigger an error, it fails an opportunity to rewrite a GOT
access.
In a Linux Kernel build this causes ~15k GOT accesses using lgrl to
be skipped to be rewritten to larl.
Resolve both issues by simply checking whether the symbol address is
halfword aligned. Do not check the symbol value nor section defining
the symbol for halfword alignment.
bfd/
PR ld/32969
* elf64-s390.c (elf_s390_relocate_section): Only rewrite
lgrl/lg from GOT to larl if symbol address is halfword aligned.
ld/testsuite/
PR ld/32969
* ld-s390/s390.exp (pr32969_64-1, pr32969_64-2): Add tests for
rewrite of GOT access when COMDAT section deduplication is
involved.
* ld-s390/pr32969_64-1.dd: New test for rewrite of GOT access
when COMDAT section deduplication is involved.
* ld-s390/pr32969_64-2.dd: Likewise.
* ld-s390/pr32969a.s: Likewise.
* ld-s390/pr32969b.s: Likewise.
* ld-s390/pr32969c.s: Likewise.
Bug: https://sourceware.org/PR32969
Fixes: e6213e09ed0e ("S/390: Prevent GOT access rewrite for certain symbols")
Reported-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
7 files changed
