diff: avoid segfault with freed entries

From: Derrick Stolee <[email protected]>

When computing a diff in a partial clone, there is a chance that we could trigger a prefetch of missing objects at the same time as we are freeing entries from the global diff queue. This is difficult to reproduce, as we need to have some objects be freed from the queue before triggering the prefetch of missing objects. There is a new test in t4067 that does trigger the segmentation fault that results in this case.

The fix is to set the queue pointer to NULL after it is freed, and then to be careful about NULL values in the prefetch.

The more elaborate explanation is that within diffcore_std(), we may skip the initial prefetch due to the output format (--name-only in the test) and go straight to diffcore_skip_stat_unmatch(). In that method, the index entries that have been invalidated by path changes show up as entries but may be deleted because they are not actually content diffs and only newer timestamps than expected. As those entries are deleted, later entries are checked with diff_filespec_check_stat_unmatch(), which uses diff_queued_diff_prefetch() as the missing_object_cb in its diff options. That can trigger downloading missing objects if the appropriate scenario occurs to trigger a call to diff_popoulate_filespec(). It's finally within that callback to diff_queued_diff_prefetch() that the segfault occurs.

The test was hard to find because it required some real differences, some not-different files that had a newer modified time, and the order of those files alphabetically was important to trigger the deletion before the prefetch was triggered.

I briefly considered a "lock" member for the diff queue, but it was a much larger diff and introduced many more possible error scenarios.

---
diff: avoid segfault with freed entries

I found this segfault in the wild in a pipeline that was using the microsoft/git fork, but the error can be reproduced without that fork. It requires partial clone, though.

Thanks, -Stolee

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2027%2Fderrickstolee%2Fdiff-segfault-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2027/derrickstolee/diff-segfault-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2027

 diff.c                        |  5 +++++
 t/t4067-diff-partial-clone.sh | 35 +++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+)

index a1961526c0..72661d635c 100644
@@ -7046,6 +7046,7 @@ static void diffcore_skip_stat_unmatch(struct diff_options *diffopt)
 			if (!diffopt->flags.no_index)
 				diffopt->skip_stat_unmatch++;
 			diff_free_filepair(p);
+			q->queue[i] = NULL;
 		}
 	}
 	free(q->queue);

@@ -7089,6 +7090,10 @@ void diff_queued_diff_prefetch(void *repository)
 
 	for (i = 0; i < q->nr; i++) {
 		struct diff_filepair *p = q->queue[i];
+
+		if (!p)
+			continue;
+
 		diff_add_if_missing(repo, &to_fetch, p->one);
 		diff_add_if_missing(repo, &to_fetch, p->two);
 	}

index 581250dd2d..72f25de449 100755
@@ -132,6 +132,41 @@ test_expect_success 'diff with rename detection batches blobs' '
 	test_line_count = 1 done_lines
 '
 
+test_expect_success 'diff succeeds even if entries are removed from queue' '
+	test_when_finished "rm -rf server client trace" &&
+
+	test_create_repo server &&
+	for l in a c e g i p
+	do
+		echo $l >server/$l &&
+		git -C server add $l || return 1
+	done &&
+	git -C server commit -m x &&
+
+	for l in a e i
+	do
+		git -C server rm $l || return 1
+	done &&
+
+	for l in b d f i
+		do
+		echo $l$l >server/$l &&
+		git -C server add $l || return 1
+	done &&
+	git -C server commit -a -m x &&
+
+	test_config -C server uploadpack.allowfilter 1 &&
+	test_config -C server uploadpack.allowanysha1inwant 1 &&
+	git clone --filter=blob:limit=0 "file://$(pwd)/server" client &&
+
+	for file in $(ls client)
+	do
+		cat client/$file >$file &&
+		mv $file client/$file || return 1
+	done &&
+	git -C client diff --name-only --relative HEAD^
+'
+
 test_expect_success 'diff does not fetch anything if inexact rename detection is not needed' '
 	test_when_finished "rm -rf server client trace" &&
 

base-commit: 9a2fb147f2c61d0cab52c883e7e26f5b7948e3ed

gitgitgadget

That's very interesting. We have code to fetch on-demand when it turns out that the initial prefetch shouldn't have been skipped and we need contents, so in that sense, the condition to skip the initial prefetch does not have to be precise, but we may want to see if we can have a single helper function that exactly tells us if we need to look at the contents or not. I think we had a few changes that made the definition of "diff status is based on contents" in the past few releases, not for the purpose of this prefetch skipping but to set the exit status.

s/diff_popoulate_filespec/diff_populate_filespec/