[PATCH 2/3] split-index: use oideq instead of memcmp to compare object_id's

["Andrzej Hunt via GitGitGadget" <gitgitgadget@gmail.com>]

From: Andrzej Hunt <ajrhunt@google.com>

cache_entry contains an object_id, and compare_ce_content() would
include that field when calling memcmp on a subset of the cache_entry.
Depending on which hashing algorithm is being used, only part of
object_id.hash is actually being used, therefore including it in a
memcmp() is technically incorrect. Instead we choose to exclude the
object_id when calling memcmp(), and call oideq() separately.

This issue was found when running t1700-split-index with MSAN, see MSAN
output below (on my machine, offset 76 corresponds to 4 bytes after the
start of object_id.hash).

Uninitialized bytes in MemcmpInterceptorCommon at offset 76 inside [0x7f60e7c00118, 92)
==27914==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x4524ee in memcmp /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/msan/../sanitizer_common/sanitizer_common_interceptors.inc:873:10
    #1 0xc867ae in compare_ce_content /home/ahunt/git/git/split-index.c:208:8
    #2 0xc859fb in prepare_to_write_split_index /home/ahunt/git/git/split-index.c:336:9
    #3 0xb4bbca in write_split_index /home/ahunt/git/git/read-cache.c:3107:2
    #4 0xb42b4d in write_locked_index /home/ahunt/git/git/read-cache.c:3295:8
    #5 0x638058 in try_merge_strategy /home/ahunt/git/git/builtin/merge.c:758:7
    #6 0x63057f in cmd_merge /home/ahunt/git/git/builtin/merge.c:1663:9
    #7 0x4a1e76 in run_builtin /home/ahunt/git/git/git.c:461:11
    #8 0x49e1e7 in handle_builtin /home/ahunt/git/git/git.c:714:3
    #9 0x4a0c08 in run_argv /home/ahunt/git/git/git.c:781:4
    #10 0x49d5a8 in cmd_main /home/ahunt/git/git/git.c:912:19
    #11 0x7974da in main /home/ahunt/git/git/common-main.c:52:11
    #12 0x7f60e928e349 in __libc_start_main (/lib64/libc.so.6+0x24349)
    #13 0x421bd9 in _start /home/abuild/rpmbuild/BUILD/glibc-2.26/csu/../sysdeps/x86_64/start.S:120

  Uninitialized value was stored to memory at
    #0 0x447eb9 in __msan_memcpy /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/msan/msan_interceptors.cpp:1558:3
    #1 0xb4d1e6 in dup_cache_entry /home/ahunt/git/git/read-cache.c:3457:2
    #2 0xd214fa in add_entry /home/ahunt/git/git/unpack-trees.c:215:18
    #3 0xd1fae0 in keep_entry /home/ahunt/git/git/unpack-trees.c:2276:2
    #4 0xd1ff9e in twoway_merge /home/ahunt/git/git/unpack-trees.c:2504:11
    #5 0xd27028 in call_unpack_fn /home/ahunt/git/git/unpack-trees.c:593:12
    #6 0xd2443d in unpack_nondirectories /home/ahunt/git/git/unpack-trees.c:1106:12
    #7 0xd19435 in unpack_callback /home/ahunt/git/git/unpack-trees.c:1306:6
    #8 0xd0d7ff in traverse_trees /home/ahunt/git/git/tree-walk.c:532:17
    #9 0xd1773a in unpack_trees /home/ahunt/git/git/unpack-trees.c:1683:9
    #10 0xdc6370 in checkout /home/ahunt/git/git/merge-ort.c:3590:8
    #11 0xdc51c3 in merge_switch_to_result /home/ahunt/git/git/merge-ort.c:3728:7
    #12 0xa195a9 in merge_ort_recursive /home/ahunt/git/git/merge-ort-wrappers.c:58:2
    #13 0x637fff in try_merge_strategy /home/ahunt/git/git/builtin/merge.c:751:12
    #14 0x63057f in cmd_merge /home/ahunt/git/git/builtin/merge.c:1663:9
    #15 0x4a1e76 in run_builtin /home/ahunt/git/git/git.c:461:11
    #16 0x49e1e7 in handle_builtin /home/ahunt/git/git/git.c:714:3
    #17 0x4a0c08 in run_argv /home/ahunt/git/git/git.c:781:4
    #18 0x49d5a8 in cmd_main /home/ahunt/git/git/git.c:912:19
    #19 0x7974da in main /home/ahunt/git/git/common-main.c:52:11

  Uninitialized value was created by a heap allocation
    #0 0x44e73d in malloc /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/msan/msan_interceptors.cpp:901:3
    #1 0xd592f6 in do_xmalloc /home/ahunt/git/git/wrapper.c:41:8
    #2 0xd59248 in xmalloc /home/ahunt/git/git/wrapper.c:62:9
    #3 0xa17088 in mem_pool_alloc_block /home/ahunt/git/git/mem-pool.c:22:6
    #4 0xa16f78 in mem_pool_init /home/ahunt/git/git/mem-pool.c:44:3
    #5 0xb481b8 in load_all_cache_entries /home/ahunt/git/git/read-cache.c
    #6 0xb44d40 in do_read_index /home/ahunt/git/git/read-cache.c:2298:17
    #7 0xb48a1b in read_index_from /home/ahunt/git/git/read-cache.c:2389:8
    #8 0xbd5a0b in repo_read_index /home/ahunt/git/git/repository.c:276:8
    #9 0xb4bcaf in repo_read_index_unmerged /home/ahunt/git/git/read-cache.c:3326:2
    #10 0x62ed26 in cmd_merge /home/ahunt/git/git/builtin/merge.c:1362:6
    #11 0x4a1e76 in run_builtin /home/ahunt/git/git/git.c:461:11
    #12 0x49e1e7 in handle_builtin /home/ahunt/git/git/git.c:714:3
    #13 0x4a0c08 in run_argv /home/ahunt/git/git/git.c:781:4
    #14 0x49d5a8 in cmd_main /home/ahunt/git/git/git.c:912:19
    #15 0x7974da in main /home/ahunt/git/git/common-main.c:52:11
    #16 0x7f60e928e349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: MemorySanitizer: use-of-uninitialized-value /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/msan/../sanitizer_common/sanitizer_common_interceptors.inc:873:10 in memcmp
Exiting

Signed-off-by: Andrzej Hunt <andrzej@ahunt.org>
---
 split-index.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/split-index.c b/split-index.c
index 4d6e52d46f75..8e52e891c3bc 100644
--- a/split-index.c
+++ b/split-index.c
@@ -207,7 +207,8 @@ static int compare_ce_content(struct cache_entry *a, struct cache_entry *b)
 	b->ce_flags &= ondisk_flags;
 	ret = memcmp(&a->ce_stat_data, &b->ce_stat_data,
 		     offsetof(struct cache_entry, name) -
-		     offsetof(struct cache_entry, ce_stat_data));
+		     offsetof(struct cache_entry, oid)) ||
+		!oideq(&a->oid, &b->oid);
 	a->ce_flags = ce_flags;
 	b->ce_flags = base_flags;
 
-- 
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