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From: "avarab at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug analyzer/105285] False positive with
 -Wanalyzer-null-dereference in git.git's reftable/reader.c
Date: Fri, 15 Apr 2022 09:49:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105285

--- Comment #1 from =C3=86var Arnfj=C3=B6r=C3=B0 Bjarmason <avarab at gmail=
 dot com> ---
Created attachment 52814
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D52814&action=3Dedit
A patch to git.git that works around the -fanalyzer false positive

A fix to git.git to work around the analyzer false positive, this diff on t=
op
shows an annotated path through the program it thinks we'd take. 24 and 28
can't be true/false false/true, only false/false true/true.

diff --git a/reftable/reader.c b/reftable/reader.c
index ea66771165f..d3a4639d6ca 100644
--- a/reftable/reader.c
+++ b/reftable/reader.c
@@ -61,6 +61,9 @@ static int reader_get_block(struct reftable_block *dest,
                            uint32_t sz, const uint64_t off,
                            const uint64_t r_size)
 {
+       if (off >=3D r_size) /* (28) following =E2=80=98true=E2=80=99 branc=
h (when =E2=80=98off >=3D
r_size=E2=80=99)... */
+               return 0;
+
        if (off + sz > r_size) {
                sz =3D r_size - off;
        }
@@ -288,13 +291,14 @@ int reader_init_block_reader(struct reftable_reader *=
r,
struct block_reader *br,
        uint32_t header_off =3D next_off ? 0 : header_size(r->version);
        int32_t block_size =3D 0;

-       if (next_off >=3D r_size)
+       if (next_off >=3D r_size) /* (24) following =E2=80=98false=E2=80=99=
 branch (when
=E2=80=98next_off < r_size=E2=80=99)... */
                return 1;

        err =3D reader_get_block(&block, &r->source, next_off, guess_block_=
size,
r_size);
-       if (err < 0)
+       if (err < 0) /* (31) following =E2=80=98false=E2=80=99 branch (when=
 =E2=80=98err >=3D 0=E2=80=99)... */
                goto done;

+       /* We'll supposedly deference NULL block.data[0] here ... */=20
        block_size =3D extract_block_size(block.data, &block_typ, next_off,
                                        r->version);
        if (block_size < 0) {=