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pack-bitmap.c: avoid uninitialized pack_int_id during reuse

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When performing multi-pack reuse, reuse_partial_packfile_from_bitmap()
is responsible for generating an array of bitmapped_pack structs from
which to perform reuse.

In the multi-pack case, we loop over the MIDXs packs and copy the result
of calling `nth_bitmapped_pack()` to construct the list of reusable
paths.

But we may also want to do pack-reuse over a single pack, either because
we only had one pack to perform reuse over (in the case of single-pack
bitmaps), or because we explicitly asked to do single pack reuse even
with a MIDX[^1].

When this is the case, the array we generate of reusable packs contains
only a single element, which is either (a) the pack attached to the
single-pack bitmap, or (b) the MIDX's preferred pack.

In 795006fff4 (pack-bitmap: gracefully handle missing BTMP chunks,
2024-04-15), we refactored the reuse_partial_packfile_from_bitmap()
function and stopped assigning the pack_int_id field when reusing only
the MIDX's preferred pack. This results in an uninitialized read down in
try_partial_reuse() like so:

    ==7474==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x55c5cd191dde in try_partial_reuse pack-bitmap.c:1887:8
    #1 0x55c5cd191dde in reuse_partial_packfile_from_bitmap_1 pack-bitmap.c:2001:8
    #2 0x55c5cd191dde in reuse_partial_packfile_from_bitmap pack-bitmap.c:2105:3
    #3 0x55c5cce0bd0e in get_object_list_from_bitmap builtin/pack-objects.c:4043:3
    #4 0x55c5cce0bd0e in get_object_list builtin/pack-objects.c:4156:27
    #5 0x55c5cce0bd0e in cmd_pack_objects builtin/pack-objects.c:4596:3
    #6 0x55c5ccc8fac8 in run_builtin git.c:474:11

which happens when try_partial_reuse() tries to call
midx_pair_to_pack_pos() when it tries to reject cross-pack deltas.

Avoid the uninitialized read by ensuring that the pack_int_id field is
set in the single-pack reuse case by setting it to either the MIDX
preferred pack's pack_int_id, or '-1', in the case of single-pack
bitmaps.  In the latter case, we never read the pack_int_id field, so
the choice of '-1' is intentional as a "garbage in, garbage out"
measure.

Guard against further regressions in this area by adding a test which
ensures that we do not throw out deltas from the preferred pack as
"cross-pack" due to an uninitialized pack_int_id.

[^1]: This can happen for a couple of reasons, either because the
  repository is configured with 'pack.allowPackReuse=(true|single)', or
  because the MIDX was generated prior to the introduction of the BTMP
  chunk, which contains information necessary to perform multi-pack
  reuse.

Reported-by: Kyle Lippincott <spectral@google.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Taylor Blau 2024-06-11 13:28:20 -04:00 committed by Junio C Hamano
parent 0c5a62f14b
commit ed4a1d6ae1
2 changed files with 39 additions and 0 deletions
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13
pack-bitmap.c
View file

@ -2073,6 +2073,7 @@ void reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
QSORT(packs, packs_nr, bitmapped_pack_cmp);
} else {
struct packed_git *pack;
uint32_t pack_int_id;
if (bitmap_is_midx(bitmap_git)) {
uint32_t preferred_pack_pos;
@ -2083,12 +2084,24 @@ void reuse_partial_packfile_from_bitmap(struct bitmap_index *bitmap_git,
}
pack = bitmap_git->midx->packs[preferred_pack_pos];
pack_int_id = preferred_pack_pos;
} else {
pack = bitmap_git->pack;
/*
* Any value for 'pack_int_id' will do here. When we
* process the pack via try_partial_reuse(), we won't
* use the `pack_int_id` field since we have a non-MIDX
* bitmap.
*
* Use '-1' as a sentinel value to make it clear
* that we do not expect to read this field.
*/
pack_int_id = -1;
}
ALLOC_GROW(packs, packs_nr + 1, packs_alloc);
packs[packs_nr].p = pack;
packs[packs_nr].pack_int_id = pack_int_id;
packs[packs_nr].bitmap_nr = pack->num_objects;
packs[packs_nr].bitmap_pos = 0;

26
t/t5332-multi-pack-reuse.sh
View file

@ -204,4 +204,30 @@ test_expect_success 'omit delta from uninteresting base (cross pack)' '
test_pack_objects_reused_all $(($objects_nr - 1)) $packs_nr
'
test_expect_success 'non-omitted delta in MIDX preferred pack' '
test_config pack.allowPackReuse single &&
cat >p1.objects <<-EOF &&
$(git rev-parse $base)
^$(git rev-parse $delta^)
EOF
cat >p2.objects <<-EOF &&
$(git rev-parse F)
EOF
p1="$(git pack-objects --revs $packdir/pack <p1.objects)" &&
p2="$(git pack-objects --revs $packdir/pack <p2.objects)" &&
cat >in <<-EOF &&
pack-$p1.idx
pack-$p2.idx
EOF
git multi-pack-index write --bitmap --stdin-packs \
--preferred-pack=pack-$p1.pack <in &&
git show-index <$packdir/pack-$p1.idx >expect &&
test_pack_objects_reused_all $(wc -l <expect) 1
'
test_done