From: Derrick Stolee Date: Mon, 26 Feb 2018 14:06:27 GMT Subject: Re: Use of uninitialised value of size 8 in sha1_name.c Message-ID: <0a85ea3b-3f64-f67d-b4d5-a761cbc4c6db@gmail.com> In-Reply-To: On 2/26/2018 5:23 AM, Christian Couder wrote: > On Mon, Feb 26, 2018 at 10:53 AM, Jeff King wrote: >> On Mon, Feb 26, 2018 at 10:04:22AM +0100, Christian Couder wrote: >> >>> ==21455== Use of uninitialised value of size 8 >>> ==21455== at 0x2D2A73: get_hex_char_from_oid (sha1_name.c:492) >>> ==21455== by 0x2D2AFE: extend_abbrev_len (sha1_name.c:502) >>> ==21455== by 0x2D2C3D: find_abbrev_len_for_pack (sha1_name.c:551) >>> ==21455== by 0x2D2CFF: find_abbrev_len_packed (sha1_name.c:569) >>> ==21455== by 0x2D2E12: find_unique_abbrev_r (sha1_name.c:608) >>> ==21455== by 0x2DCB66: strbuf_add_unique_abbrev (strbuf.c:877) >>> ==21455== by 0x14F7CE: update_local_ref (fetch.c:700) >>> ==21455== by 0x1500CF: store_updated_refs (fetch.c:871) >>> ==21455== by 0x15035B: fetch_refs (fetch.c:932) >>> ==21455== by 0x150CF8: do_fetch (fetch.c:1146) >>> ==21455== by 0x1515AB: fetch_one (fetch.c:1370) >>> ==21455== by 0x151A1D: cmd_fetch (fetch.c:1457) >>> ==21455== Uninitialised value was created by a stack allocation >>> ==21455== at 0x2D2B2E: find_abbrev_len_for_pack (sha1_name.c:513) >>> ==21455== >>> >>> A quick git blame seems to point to 0e87b85683 (sha1_name: minimize >>> OID comparisons during disambiguation, 2017-10-12). >>> >>> It is difficult to tell for sure though as t5616-partial-clone.sh was >>> added after that commit. >> I think that commit is to blame, though the error isn't exactly where >> that stack trace puts it. Try this: >> >> diff --git a/sha1_name.c b/sha1_name.c >> index 611c7d24dd..6f7f36436f 100644 >> --- a/sha1_name.c >> +++ b/sha1_name.c >> @@ -547,7 +547,10 @@ static void find_abbrev_len_for_pack(struct packed_git *p, >> */ >> mad->init_len = 0; >> if (!match) { >> - nth_packed_object_oid(&oid, p, first); >> + warning("p->num_objects = %u, first = %u", >> + p->num_objects, first); >> + if (!nth_packed_object_oid(&oid, p, first)) >> + die("oops!"); >> extend_abbrev_len(&oid, mad); >> } else if (first < num - 1) { >> nth_packed_object_oid(&oid, p, first + 1); >> >> I get failures all over the test suite, like this: >> >> warning: p->num_objects = 4, first = 3 >> warning: p->num_objects = 8, first = 6 >> warning: p->num_objects = 10, first = 0 >> warning: p->num_objects = 4, first = 0 >> warning: p->num_objects = 8, first = 0 >> warning: p->num_objects = 10, first = 10 >> fatal: oops! > Yeah, I tried to t5601-clone.sh with --valgrind and I also get one > error (the same "Uninitialised value" error actually). Thanks for finding this. Sorry for the bug. >> Any time the abbreviated hex would go after the last object in the pack, >> then first==p->num_objects, and nth_packed_object_oid() will fail. That >> leaves uninitialized data in "oid", which is what valgrind complains >> about when we examine it in extend_abbrev_len(). >> >> Most of the time the code behaves correctly anyway, because the >> uninitialized bytes are unlikely to match up with our hex and extend the >> length. Probably we just need to detect that case and skip the call to >> extend_abbrev_len() altogether. > Yeah, something like: > > diff --git a/sha1_name.c b/sha1_name.c > index 611c7d24dd..a041d8d24f 100644 > --- a/sha1_name.c > +++ b/sha1_name.c > @@ -547,15 +547,15 @@ static void find_abbrev_len_for_pack(struct packed_git *p, > */ > mad->init_len = 0; > if (!match) { > - nth_packed_object_oid(&oid, p, first); > - extend_abbrev_len(&oid, mad); > + if (nth_packed_object_oid(&oid, p, first)) > + extend_abbrev_len(&oid, mad); > } else if (first < num - 1) { > - nth_packed_object_oid(&oid, p, first + 1); > - extend_abbrev_len(&oid, mad); > + if (nth_packed_object_oid(&oid, p, first + 1)) > + extend_abbrev_len(&oid, mad); > } > if (first > 0) { > - nth_packed_object_oid(&oid, p, first - 1); > - extend_abbrev_len(&oid, mad); > + if (nth_packed_object_oid(&oid, p, first - 1)) > + extend_abbrev_len(&oid, mad); > } > mad->init_len = mad->cur_len; > } > > seems to prevent valgrind from complaining when running t5616-partial-clone.sh. This seems like the safest fix, but also we could use our values of "match", "first" and "num" to safely call nth_packed_object_oid(). However, since nth_packed_object_oid() checks the bounds, don't duplicate work and just use the return value. I would reformat your patch slightly, but that's just preference: diff --git a/sha1_name.c b/sha1_name.c index 611c7d2..97b632c 100644 --- a/sha1_name.c +++ b/sha1_name.c @@ -546,17 +546,14 @@ static void find_abbrev_len_for_pack(struct packed_git *p,          * nearby for the abbreviation length.          */         mad->init_len = 0; -       if (!match) { -               nth_packed_object_oid(&oid, p, first); +       if (!match && nth_packed_object_oid(&oid, p, first))                 extend_abbrev_len(&oid, mad); -       } else if (first < num - 1) { -               nth_packed_object_oid(&oid, p, first + 1); +       else if (first < num - 1 && +                nth_packed_object_oid(&oid, p, first + 1))                 extend_abbrev_len(&oid, mad); -       } -       if (first > 0) { -               nth_packed_object_oid(&oid, p, first - 1); +       if (first > 0 && nth_packed_object_oid(&oid, p, first - 1))                 extend_abbrev_len(&oid, mad); -       } +         mad->init_len = mad->cur_len;  } Christian: do you want to submit the patch, or should I put one together? Thanks, -Stolee