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/* Warn on problematic uses of alloca and variable length arrays.
Copyright (C) 2016-2021 Free Software Foundation, Inc.
Contributed by Aldy Hernandez <aldyh@redhat.com>.
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify it under
the terms of the GNU General Public License as published by the Free
Software Foundation; either version 3, or (at your option) any later
version.
GCC is distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
for more details.
You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING3. If not see
<http://www.gnu.org/licenses/>. */
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "backend.h"
#include "tree.h"
#include "gimple.h"
#include "tree-pass.h"
#include "ssa.h"
#include "gimple-pretty-print.h"
#include "diagnostic-core.h"
#include "fold-const.h"
#include "gimple-iterator.h"
#include "tree-ssa.h"
#include "tree-cfg.h"
#include "builtins.h"
#include "calls.h"
#include "cfgloop.h"
#include "intl.h"
#include "gimple-range.h"
static unsigned HOST_WIDE_INT adjusted_warn_limit (bool);
const pass_data pass_data_walloca = {
GIMPLE_PASS,
"walloca",
OPTGROUP_NONE,
TV_NONE,
PROP_cfg, // properties_required
0, // properties_provided
0, // properties_destroyed
0, // properties_start
0, // properties_finish
};
class pass_walloca : public gimple_opt_pass
{
public:
pass_walloca (gcc::context *ctxt)
: gimple_opt_pass(pass_data_walloca, ctxt), first_time_p (false)
{}
opt_pass *clone () { return new pass_walloca (m_ctxt); }
void set_pass_param (unsigned int n, bool param)
{
gcc_assert (n == 0);
first_time_p = param;
}
virtual bool gate (function *);
virtual unsigned int execute (function *);
private:
// Set to TRUE the first time we run this pass on a function.
bool first_time_p;
};
bool
pass_walloca::gate (function *fun ATTRIBUTE_UNUSED)
{
// The first time this pass is called, it is called before
// optimizations have been run and range information is unavailable,
// so we can only perform strict alloca checking.
if (first_time_p)
return warn_alloca != 0;
// Warning is disabled when its size limit is greater than PTRDIFF_MAX
// for the target maximum, which makes the limit negative since when
// represented in signed HOST_WIDE_INT.
unsigned HOST_WIDE_INT max = tree_to_uhwi (TYPE_MAX_VALUE (ptrdiff_type_node));
return (adjusted_warn_limit (false) <= max
|| adjusted_warn_limit (true) <= max);
}
// Possible problematic uses of alloca.
enum alloca_type {
// Alloca argument is within known bounds that are appropriate.
ALLOCA_OK,
// Alloca argument is KNOWN to have a value that is too large.
ALLOCA_BOUND_DEFINITELY_LARGE,
// Alloca argument may be too large.
ALLOCA_BOUND_MAYBE_LARGE,
// Alloca appears in a loop.
ALLOCA_IN_LOOP,
// Alloca argument is 0.
ALLOCA_ARG_IS_ZERO,
// Alloca call is unbounded. That is, there is no controlling
// predicate for its argument.
ALLOCA_UNBOUNDED
};
// Type of an alloca call with its corresponding limit, if applicable.
class alloca_type_and_limit {
public:
enum alloca_type type;
// For ALLOCA_BOUND_MAYBE_LARGE and ALLOCA_BOUND_DEFINITELY_LARGE
// types, this field indicates the assumed limit if known or
// integer_zero_node if unknown. For any other alloca types, this
// field is undefined.
wide_int limit;
alloca_type_and_limit ();
alloca_type_and_limit (enum alloca_type type,
wide_int i) : type(type), limit(i) { }
alloca_type_and_limit (enum alloca_type type) : type(type)
{
limit = wi::to_wide (integer_zero_node);
}
};
/* Return TRUE if the user specified a limit for either VLAs or ALLOCAs. */
static bool
warn_limit_specified_p (bool is_vla)
{
unsigned HOST_WIDE_INT max = is_vla ? warn_vla_limit : warn_alloca_limit;
return max != HOST_WIDE_INT_MAX;
}
/* Return the value of the argument N to -Walloca-larger-than= or
-Wvla-larger-than= adjusted for the target data model so that
when N == HOST_WIDE_INT_MAX, the adjusted value is set to
PTRDIFF_MAX on the target. This is done to prevent warnings
for unknown/unbounded allocations in the "permissive mode"
while still diagnosing excessive and necessarily invalid
allocations. */
static unsigned HOST_WIDE_INT
adjusted_warn_limit (bool idx)
{
static HOST_WIDE_INT limits[2];
if (limits[idx])
return limits[idx];
limits[idx] = idx ? warn_vla_limit : warn_alloca_limit;
if (limits[idx] != HOST_WIDE_INT_MAX)
return limits[idx];
limits[idx] = tree_to_shwi (TYPE_MAX_VALUE (ptrdiff_type_node));
return limits[idx];
}
// Analyze the alloca call in STMT and return the alloca type with its
// corresponding limit (if applicable). IS_VLA is set if the alloca
// call was created by the gimplifier for a VLA.
static class alloca_type_and_limit
alloca_call_type (range_query &query, gimple *stmt, bool is_vla)
{
gcc_assert (gimple_alloca_call_p (stmt));
tree len = gimple_call_arg (stmt, 0);
gcc_assert (!is_vla || warn_vla_limit >= 0);
gcc_assert (is_vla || warn_alloca_limit >= 0);
// Adjust warn_alloca_max_size for VLAs, by taking the underlying
// type into account.
unsigned HOST_WIDE_INT max_size = adjusted_warn_limit (is_vla);
// Check for the obviously bounded case.
if (TREE_CODE (len) == INTEGER_CST)
{
if (tree_to_uhwi (len) > max_size)
return alloca_type_and_limit (ALLOCA_BOUND_DEFINITELY_LARGE,
wi::to_wide (len));
if (integer_zerop (len))
{
const offset_int maxobjsize
= wi::to_offset (max_object_size ());
alloca_type result = (max_size < maxobjsize
? ALLOCA_ARG_IS_ZERO : ALLOCA_OK);
return alloca_type_and_limit (result);
}
return alloca_type_and_limit (ALLOCA_OK);
}
struct alloca_type_and_limit ret = alloca_type_and_limit (ALLOCA_OK);
// If we have a declared maximum size, we can take it into account.
if (gimple_call_builtin_p (stmt, BUILT_IN_ALLOCA_WITH_ALIGN_AND_MAX))
{
tree arg = gimple_call_arg (stmt, 2);
if (compare_tree_int (arg, max_size) <= 0)
ret = alloca_type_and_limit (ALLOCA_OK);
else
{
const offset_int maxobjsize
= wi::to_offset (max_object_size ());
alloca_type result = (max_size < maxobjsize
? ALLOCA_BOUND_MAYBE_LARGE : ALLOCA_OK);
ret = alloca_type_and_limit (result, wi::to_wide (arg));
}
return ret;
}
// If the user specified a limit, use it.
int_range_max r;
if (warn_limit_specified_p (is_vla)
&& TREE_CODE (len) == SSA_NAME
&& query.range_of_expr (r, len, stmt)
&& !r.varying_p ())
{
// The invalid bits are anything outside of [0, MAX_SIZE].
static int_range<2> invalid_range (build_int_cst (size_type_node, 0),
build_int_cst (size_type_node,
max_size),
VR_ANTI_RANGE);
r.intersect (invalid_range);
if (r.undefined_p ())
return alloca_type_and_limit (ALLOCA_OK);
return alloca_type_and_limit (ALLOCA_BOUND_MAYBE_LARGE,
wi::to_wide (integer_zero_node));
}
const offset_int maxobjsize = tree_to_shwi (max_object_size ());
/* When MAX_SIZE is greater than or equal to PTRDIFF_MAX treat
allocations that aren't visibly constrained as OK, otherwise
report them as (potentially) unbounded. */
alloca_type unbounded_result = (max_size < maxobjsize.to_uhwi ()
? ALLOCA_UNBOUNDED : ALLOCA_OK);
return alloca_type_and_limit (unbounded_result);
}
// Return TRUE if STMT is in a loop, otherwise return FALSE.
static bool
in_loop_p (gimple *stmt)
{
basic_block bb = gimple_bb (stmt);
return
bb->loop_father && bb->loop_father->header != ENTRY_BLOCK_PTR_FOR_FN (cfun);
}
unsigned int
pass_walloca::execute (function *fun)
{
gimple_ranger ranger;
basic_block bb;
FOR_EACH_BB_FN (bb, fun)
{
for (gimple_stmt_iterator si = gsi_start_bb (bb); !gsi_end_p (si);
gsi_next (&si))
{
gimple *stmt = gsi_stmt (si);
if (!gimple_alloca_call_p (stmt))
continue;
location_t loc = gimple_nonartificial_location (stmt);
loc = expansion_point_location_if_in_system_header (loc);
const bool is_vla
= gimple_call_alloca_for_var_p (as_a <gcall *> (stmt));
// Strict mode whining for VLAs is handled by the front-end,
// so we can safely ignore this case. Also, ignore VLAs if
// the user doesn't care about them.
if (is_vla)
{
if (warn_vla > 0 || warn_vla_limit < 0)
continue;
}
else if (warn_alloca)
{
warning_at (loc, OPT_Walloca, "%Guse of %<alloca%>", stmt);
continue;
}
else if (warn_alloca_limit < 0)
continue;
class alloca_type_and_limit t
= alloca_call_type (ranger, stmt, is_vla);
unsigned HOST_WIDE_INT adjusted_alloca_limit
= adjusted_warn_limit (false);
// Even if we think the alloca call is OK, make sure it's not in a
// loop, except for a VLA, since VLAs are guaranteed to be cleaned
// up when they go out of scope, including in a loop.
if (t.type == ALLOCA_OK && !is_vla && in_loop_p (stmt))
{
/* As in other instances, only diagnose this when the limit
is less than the maximum valid object size. */
const offset_int maxobjsize
= wi::to_offset (max_object_size ());
if (adjusted_alloca_limit < maxobjsize.to_uhwi ())
t = alloca_type_and_limit (ALLOCA_IN_LOOP);
}
enum opt_code wcode
= is_vla ? OPT_Wvla_larger_than_ : OPT_Walloca_larger_than_;
char buff[WIDE_INT_MAX_PRECISION / 4 + 4];
switch (t.type)
{
case ALLOCA_OK:
break;
case ALLOCA_BOUND_MAYBE_LARGE:
{
auto_diagnostic_group d;
if (warning_at (loc, wcode,
(is_vla
? G_("%Gargument to variable-length "
"array may be too large")
: G_("%Gargument to %<alloca%> may be too "
"large")),
stmt)
&& t.limit != 0)
{
print_decu (t.limit, buff);
inform (loc, "limit is %wu bytes, but argument "
"may be as large as %s",
is_vla ? warn_vla_limit : adjusted_alloca_limit,
buff);
}
}
break;
case ALLOCA_BOUND_DEFINITELY_LARGE:
{
auto_diagnostic_group d;
if (warning_at (loc, wcode,
(is_vla
? G_("%Gargument to variable-length"
" array is too large")
: G_("%Gargument to %<alloca%> is too large")),
stmt)
&& t.limit != 0)
{
print_decu (t.limit, buff);
inform (loc, "limit is %wu bytes, but argument is %s",
is_vla ? warn_vla_limit : adjusted_alloca_limit,
buff);
}
}
break;
case ALLOCA_UNBOUNDED:
warning_at (loc, wcode,
(is_vla
? G_("%Gunbounded use of variable-length array")
: G_("%Gunbounded use of %<alloca%>")),
stmt);
break;
case ALLOCA_IN_LOOP:
gcc_assert (!is_vla);
warning_at (loc, wcode,
"%Guse of %<alloca%> within a loop", stmt);
break;
case ALLOCA_ARG_IS_ZERO:
warning_at (loc, wcode,
(is_vla
? G_("%Gargument to variable-length array "
"is zero")
: G_("%Gargument to %<alloca%> is zero")),
stmt);
break;
default:
gcc_unreachable ();
}
}
}
return 0;
}
gimple_opt_pass *
make_pass_walloca (gcc::context *ctxt)
{
return new pass_walloca (ctxt);
}