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From: "dmalcolm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug analyzer/105285] False positive with
 -Wanalyzer-null-dereference in git.git's reftable/reader.c
Date: Wed, 27 Apr 2022 18:08:17 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105285

--- Comment #6 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
For (a):

If I'm reading this right:

reader_init_block_reader has:

  struct reftable_block block =3D {((void *)0)};

reader_init_block_reader checks for (next_off >=3D r->size) and bails out,
otherwise, block is passed to reader_get_block:

  if (next_off >=3D r->size)
    return 1;

  err =3D reader_get_block(r, &block, next_off, guess_block_size);
  if (err < 0)
    goto done;

  block_size =3D extract_block_size(block.data, &block_typ, next_off,
r->version);

There's an early-reject case in reader_get_block, which is:

  if (off >=3D r->size)
    return 0;

I believe the analyzer's feasibility checker is getting confused; it appear=
s to
be getting placeholder values when it access r->size, and each time it acce=
sses
r->size it gets a different placeholder value, and thus erroneously conside=
rs
the execution path where (next_off >=3D r->size) && !(off >=3D r->size) when
next_off =3D=3D off.

I'm working on a simpler reproducer, and a fix.=