From c8a45eb66e0a1086a31af4d76cd0f5e228497229 Mon Sep 17 00:00:00 2001 From: Taylor Blau Date: Wed, 25 Nov 2020 12:17:28 -0500 Subject: [PATCH] packfile.c: protect against disappearing indexes In 17c35c8969 (packfile: skip loading index if in multi-pack-index, 2018-07-12) we stopped loading the .idx file for packs that are contained within a multi-pack index. This saves us the effort of loading an .idx and doing some lightweight validity checks by way of 'packfile.c:load_idx()', but introduces a race between processes that need to load the index (e.g., to generate a reverse index) and processes that can delete the index. For example, running the following in your shell: $ git init repo && cd repo $ git commit --allow-empty -m 'base' $ git repack -ad && git multi-pack-index write followed by: $ rm -f .git/objects/pack/pack-*.idx $ git rev-parse HEAD | git cat-file --batch-check='%(objectsize:disk)' will result in a segfault prior to this patch. What's happening here is that we notice that the pack is in the multi-pack index, and so don't check that it still has a .idx. When we then try and load that index to generate a reverse index, we don't have it, so the call to 'find_pack_revindex()' in 'packfile.c:packed_object_info()' returns NULL, and then dereferencing it causes a segfault. Of course, we don't ever expect someone to remove the index file by hand, or to be in a state where we never wrote it to begin with (yet find that pack in the multi-pack-index). But, this can happen in a timing race with 'git repack -ad', which removes all existing packs after writing a new pack containing all of their objects. Avoid this by reverting the hunk of 17c35c8969 which stops loading the index when the pack is contained in a MIDX. This makes the latter half of 17c35c8969 useless, since we'll always have a non-NULL 'p->index_data', in which case that if statement isn't guarding anything. These two together effectively revert 17c35c8969, and avoid the race explained above. Co-authored-by: Jeff King Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano --- packfile.c | 19 ++----------------- t/t5319-multi-pack-index.sh | 24 ++++++++++++++++++++++-- 2 files changed, 24 insertions(+), 19 deletions(-) diff --git a/packfile.c b/packfile.c index 355066de17..eb14f7c5f1 100644 --- a/packfile.c +++ b/packfile.c @@ -514,19 +514,8 @@ static int open_packed_git_1(struct packed_git *p) ssize_t read_result; const unsigned hashsz = the_hash_algo->rawsz; - if (!p->index_data) { - struct multi_pack_index *m; - const char *pack_name = pack_basename(p); - - for (m = the_repository->objects->multi_pack_index; - m; m = m->next) { - if (midx_contains_pack(m, pack_name)) - break; - } - - if (!m && open_pack_index(p)) - return error("packfile %s index unavailable", p->pack_name); - } + if (open_pack_index(p)) + return error("packfile %s index unavailable", p->pack_name); if (!pack_max_fds) { unsigned int max_fds = get_max_fd_limit(); @@ -577,10 +566,6 @@ static int open_packed_git_1(struct packed_git *p) " supported (try upgrading GIT to a newer version)", p->pack_name, ntohl(hdr.hdr_version)); - /* Skip index checking if in multi-pack-index */ - if (!p->index_data) - return 0; - /* Verify the pack matches its index. */ if (p->num_objects != ntohl(hdr.hdr_entries)) return error("packfile %s claims to have %"PRIu32" objects" diff --git a/t/t5319-multi-pack-index.sh b/t/t5319-multi-pack-index.sh index c72ca04399..98c7c9df15 100755 --- a/t/t5319-multi-pack-index.sh +++ b/t/t5319-multi-pack-index.sh @@ -117,7 +117,7 @@ test_expect_success 'write midx with one v2 pack' ' compare_results_with_midx "one v2 pack" -test_expect_success 'corrupt idx not opened' ' +test_expect_success 'corrupt idx reports errors' ' idx=$(test-tool read-midx $objdir | grep "\.idx\$") && mv $objdir/pack/$idx backup-$idx && test_when_finished "mv backup-\$idx \$objdir/pack/\$idx" && @@ -128,7 +128,7 @@ test_expect_success 'corrupt idx not opened' ' test_copy_bytes 1064 $objdir/pack/$idx && git -c core.multiPackIndex=true rev-list --objects --all 2>err && - test_must_be_empty err + grep "index unavailable" err ' test_expect_success 'add more objects' ' @@ -547,4 +547,24 @@ test_expect_success 'repack --batch-size=0 repacks everything' ' ) ' +test_expect_success 'load reverse index when missing .idx' ' + git init repo && + test_when_finished "rm -fr repo" && + ( + cd repo && + + git config core.multiPackIndex true && + + test_commit base && + git repack -ad && + git multi-pack-index write && + + git rev-parse HEAD >tip && + idx=$(ls .git/objects/pack/pack-*.idx) && + + mv $idx $idx.bak && + git cat-file --batch-check="%(objectsize:disk)"