[Bug c/104069] New: -Werror=use-after-free false positive on elfutils-0.186

["slyfox at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104069

            Bug ID: 104069
           Summary: -Werror=use-after-free false positive on
                    elfutils-0.186
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: slyfox at gcc dot gnu.org
                CC: dmalcolm at gcc dot gnu.org
  Target Milestone: ---

Originally observed on elfutils-0.186 (which builds with -Werror by default).

Here is my attempt to extract simplified example:


typedef long unsigned int size_t;

extern void *realloc(void *__ptr, size_t __size)
    __attribute__((__nothrow__, __leaf__))
    __attribute__((__warn_unused_result__)) __attribute__((__alloc_size__(2)));

void * __libdw_unzstd(size_t todo) {
  void * sb = 0;

  for(;;) {
    // ran ony once
    if (!sb) {
      char * b = realloc(sb, todo);
      if (!b)
        break;

      sb = b;
    }

    todo -= 1;
    if (todo == 0)
      break;
  }

  // shrink buffer: leave only one byte for simplicity
  char * b = realloc(sb, 1);
  if (b) {
      sb = b;
  } else {
      // realloc failed mysteriously, leave 'sb' untouched.
  }

  return sb;
}

$ gcc-12.0.0 -O2 -std=gnu99 -Wall -Werror -c zstd.c.c
zstd.c.c: In function ‘__libdw_unzstd’:
zstd.c.c:35:10: error: pointer ‘sb’ may be used after ‘realloc’
[-Werror=use-after-free]
   35 |   return sb;
      |          ^~
zstd.c.c:28:14: note: call to ‘realloc’ here
   28 |   char * b = realloc(sb, 1);
      |              ^~~~~~~~~~~~~~
cc1: all warnings being treated as errors

I think it's a proper false positive. Original code is not as contrived (but I
think it's still correct):
https://sourceware.org/git/?p=elfutils.git;a=blob;f=libdwfl/gzip.c;h=ba8ecfba6c316b261ee38bb288ab163664ade9e5;hb=983e86fd89e8bf02f2d27ba5dce5bf078af4ceda#l180

$ gcc-12.0.0 -v
Using built-in specs.
COLLECT_GCC=/<<NIX>>/gcc-12.0.0/bin/gcc
COLLECT_LTO_WRAPPER=/<<NIX>>/gcc-12.0.0/libexec/gcc/x86_64-unknown-linux-gnu/12.0.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with:
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.0 20220116 (experimental) (GCC)