Re: Use of uninitialised value of size 8 in sha1_name.c

> On Mon, Feb 26, 2018 at 10:53 AM, Jeff King <peff@peff.net> 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).

>> 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.