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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug sanitizer/97294] ASAN "dynamic-stack-buffer-overflow" false
 positive with OpenMP reduction to std::vector
Date: Wed, 07 Oct 2020 15:56:36 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97294

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gn=
u.org
             Status|NEW                         |ASSIGNED

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 49323
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D49323&action=3Dedit
gcc11-pr97294.patch

Untested fix.
Self-contained testcase with -O0 -fsanitize=3Daddress -fopenmp could be e.g.
__attribute__((noipa)) void
foo (int *p, int n)
{
  int i;
  #pragma omp parallel for num_threads(2) reduction(+:p[:n])
  for (i =3D 0; i < 10; i++)
    {
      p[0]++;
      p[n - 1] +=3D 2;
    }
}

__attribute__((noipa)) void
bar (void)
{
  unsigned char buf[1024];
  int i;
  asm volatile ("" : : "r" (&buf[0]) : "memory");
  for (i =3D 0; i < 1024; i++)
    buf[i] =3D i;
  asm volatile ("" : : "r" (&buf[0]) : "memory");
}

int
main ()
{
  int p[50], i;
  for (i =3D 0; i < 50; i++)
    p[i] =3D 0;
  foo (p, 50);
  bar ();
  if (p[0] !=3D 10 || p[49] !=3D 20)
    __builtin_abort ();
  return 0;
}
The problem was that nothing set cfun->calls_alloca flag in the child omp
function and thus the asan code wouldn't add __asan_allocas_unpoison call at
the end of the function.  Normally when optimizing, DCE clears those flags =
and
recomputes them again, so this was only problematic with -O0.=