[Bug middle-end/106427] New: -Wuse-after-free=3 false alarm about int (not pointer) variable

[Bug middle-end/106427] New: -Wuse-after-free=3 false alarm about int (not pointer) variable

[eggert at cs dot ucla.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106427

            Bug ID: 106427
           Summary: -Wuse-after-free=3 false alarm about int (not pointer)
                    variable
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at cs dot ucla.edu
  Target Milestone: ---

Created attachment 53341
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=53341&action=edit
Compile with '-O2 -S -Wuse-after-free=3' to demonstrate the bug

I ran into this problem when building GNU Tar. Compile the attached program u.i
with "gcc -O2 -S -Wuse-after-free=3 u.i", using GCC 12.1.1 20220507 (Red Hat
12.1.1-1) on x86-64. The output (shown below) is a false alarm, because the
variable 'stop' is of type 'int', and is not a pointer. Apparently GCC is
confused by the assignment statement 'stop = p == end;' and thinks that
accessing 'stop' later implies accessing a freed pointer.

        u.i: In function ‘coalesce_segment’:
        u.i:6486:18: warning: pointer ‘p’ used after ‘free’ [-Wuse-after-free]
         6486 |   for (stop = 0; !stop;)
              |                  ^~~~~
        In function ‘wsnode_free’,
            inlined from ‘coalesce_segment’ at u.i:6499:4:
        u.i:6316:3: note: call to ‘free’ here
         6316 |   free (p);
              |   ^~~~~~~~

[Bug middle-end/106427] -Wuse-after-free=3 false alarm about int (not pointer) variable

[simon at josefsson dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106427

Simon Josefsson <simon at josefsson dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |simon at josefsson dot org

--- Comment #1 from Simon Josefsson <simon at josefsson dot org> ---
I ran into this problem too, here is a simple reproducer:

$ gcc -Werror -Wuse-after-free=3 -g -O2 -o foo foo.c
foo.c: In function 'main':
foo.c:18:6: error: pointer may be used after 'free' [-Werror=use-after-free]
   18 |   if (strstr (in, "FOO"))
      |      ^
foo.c:21:3: note: call to 'free' here
   21 |   free (in);
      |   ^~~~~~~~~
cc1: all warnings being treated as errors

#include <string.h>
#include <stdlib.h>

extern int readln (char **out);

int
main (void)
{
  char *in = NULL;
  int i = 0;

  if (!readln (&in))
    return 0;

  if (in == NULL)
    return 0;

  if (strstr (in, "FOO"))
    i = 1;

  free (in);
  in = NULL;

  if (!readln (&in))
    return 0;

  if (in == NULL)
    return 0;

  free (in);

  return i;
}