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c++: Fix get_member_function_from_ptrfunc with -fsanitize=bounds [PR116449]

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The following testcase is miscompiled, because
get_member_function_from_ptrfunc
emits something like
(((FUNCTION.__pfn & 1) != 0)
 ? ptr + FUNCTION.__delta + FUNCTION.__pfn - 1
 : FUNCTION.__pfn) (ptr + FUNCTION.__delta, ...)
or so, so FUNCTION tree is used there 5 times.  There is
if (TREE_SIDE_EFFECTS (function)) function = save_expr (function);
but in this case function doesn't have side-effects, just nested ARRAY_REFs.
Now, if all the FUNCTION trees would be shared, it would work fine,
FUNCTION is evaluated in the first operand of COND_EXPR; but unfortunately
that isn't the case, both the BIT_AND_EXPR shortening and conversion to
bool done for build_conditional_expr actually unshare_expr that first
expression, but none of the other 4 are unshared.  With -fsanitize=bounds,
.UBSAN_BOUNDS calls are added to the ARRAY_REFs and use save_expr to avoid
evaluating the argument multiple times, but because that FUNCTION tree is
first used in the second argument of COND_EXPR (i.e. conditionally), the
SAVE_EXPR initialization is done just there and then the third argument
of COND_EXPR just uses the uninitialized temporary and so does the first
argument computation as well.

The following patch fixes that by doing save_expr even if !TREE_SIDE_EFFECTS,
but to avoid doing that too often only if !nonvirtual and if the expression
isn't a simple decl.

2024-09-10  Jakub Jelinek  <jakub@redhat.com>

	PR c++/116449
	* typeck.cc (get_member_function_from_ptrfunc): Use save_expr
	on instance_ptr and function even if it doesn't have side-effects,
	as long as it isn't a decl.

	* g++.dg/ubsan/pr116449.C: New test.
This commit is contained in:
Jakub Jelinek 2024-09-10 18:32:58 +02:00 committed by Jakub Jelinek
parent 4e1e50458b
commit 0008050b9d
2 changed files with 30 additions and 3 deletions
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19
gcc/cp/typeck.cc
View File

@ -4193,10 +4193,23 @@ get_member_function_from_ptrfunc (tree *instance_ptrptr, tree function,
if (!nonvirtual && is_dummy_object (instance_ptr))
nonvirtual = true;
if (TREE_SIDE_EFFECTS (instance_ptr))
instance_ptr = instance_save_expr = save_expr (instance_ptr);
/* Use save_expr even when instance_ptr doesn't have side-effects,
unless it is a simple decl (save_expr won't do anything on
constants), so that we don't ubsan instrument the expression
multiple times. See PR116449. */
if (TREE_SIDE_EFFECTS (instance_ptr)
|| (!nonvirtual && !DECL_P (instance_ptr)))
{
instance_save_expr = save_expr (instance_ptr);
if (instance_save_expr == instance_ptr)
instance_save_expr = NULL_TREE;
else
instance_ptr = instance_save_expr;
}
if (TREE_SIDE_EFFECTS (function))
/* See above comment. */
if (TREE_SIDE_EFFECTS (function)
|| (!nonvirtual && !DECL_P (function)))
function = save_expr (function);
/* Start by extracting all the information from the PMF itself. */

14
gcc/testsuite/g++.dg/ubsan/pr116449.C Normal file
View File

@ -0,0 +1,14 @@
// PR c++/116449
// { dg-do compile }
// { dg-options "-O2 -Wall -fsanitize=undefined" }
struct C { void foo (int); void bar (); int c[16]; };
typedef void (C::*P) ();
struct D { P d; };
static D e[1] = { { &C::bar } };
void
C::foo (int x)
{
(this->*e[c[x]].d) ();
}