From 85a9a63c9268b18b24f25f6a14d6ae9966c3566d Mon Sep 17 00:00:00 2001 From: Jeff King Date: Sun, 28 Jan 2024 20:57:08 -0500 Subject: diff: handle NULL meta-info when spawning external diff Running this: $ touch foo bar $ chmod +x foo $ git -c diff.external=echo diff --ext-diff --no-index foo bar results in a segfault. The issue is that run_diff_cmd() passes a NULL "xfrm_msg" variable to run_external_diff(), which feeds it to strvec_push(), causing the segfault. The bug dates back to 82fbf269b9 (run_external_diff: use an argv_array for the command line, 2014-04-19), though it mostly only ever worked accidentally. Before then, we just stuck the NULL pointer into a "const char **" array, so our NULL ended up acting as an extra end-of-argv sentinel (which was OK, because it was the last thing in the array). Curiously, though, this is only a problem with --no-index. We set up xfrm_msg by calling fill_metainfo(). This result may be empty, or may have text like "index 1234..5678\n", "rename from foo\nrename from bar\n", etc. In run_external_diff(), we only look at xfrm_msg if the "other" variable is not NULL. That variable is set when the paths of the two sides of the diff pair aren't the same (in which case the destination path becomes "other"). So normally it would kick in only for a rename, in which case xfrm_msg should not be NULL (it would have the rename information in it). But with a "--no-index" of two blobs, we of course have two different pathnames, and thus end up with a non-NULL "other" filename (which is always just a repeat of the file2-name), but possibly a NULL xfrm_msg. So how to fix it? I have a feeling that --no-index always passing "other" to the external diff command is probably a bug. There was no rename, and the name is always redundant with existing information we pass (and this may even cause us to pass a useless "xfrm_msg" that contains an "index 1234..5678" line). So one option would be to change that behavior. We don't seem to have ever documented the "other" or "xfrm_msg" parameters for external diffs. But I'm not sure what fallout we might have from changing that behavior now. So this patch takes the less-risky option, and simply teaches run_external_diff() to avoid passing xfrm_msg when it's NULL. That makes it agnostic to whether "other" and "xfrm_msg" always come as a pair. It fixes the segfault now, and if we want to change the --no-index "other" behavior on top, it will handle that, too. Reported-by: Wilfred Hughes Signed-off-by: Jeff King Signed-off-by: Junio C Hamano --- diff.c | 3 ++- t/t4053-diff-no-index.sh | 12 ++++++++++++ 2 files changed, 14 insertions(+), 1 deletion(-) diff --git a/diff.c b/diff.c index 2c602df10a..6e066d64a5 100644 --- a/diff.c +++ b/diff.c @@ -4369,7 +4369,8 @@ static void run_external_diff(const char *pgm, add_external_diff_name(o->repo, &cmd.args, two); if (other) { strvec_push(&cmd.args, other); - strvec_push(&cmd.args, xfrm_msg); + if (xfrm_msg) + strvec_push(&cmd.args, xfrm_msg); } } diff --git a/t/t4053-diff-no-index.sh b/t/t4053-diff-no-index.sh index 5ce345d309..651ec77660 100755 --- a/t/t4053-diff-no-index.sh +++ b/t/t4053-diff-no-index.sh @@ -205,6 +205,18 @@ test_expect_success POSIXPERM,SYMLINKS 'diff --no-index normalizes: mode not lik test_cmp expected actual ' +test_expect_success POSIXPERM 'external diff with mode-only change' ' + echo content >not-executable && + echo content >executable && + chmod +x executable && + echo executable executable $(test_oid zero) 100755 \ + not-executable $(test_oid zero) 100644 not-executable \ + >expect && + test_expect_code 1 git -c diff.external=echo diff \ + --no-index executable not-executable >actual && + test_cmp expect actual +' + test_expect_success "diff --no-index treats '-' as stdin" ' cat >expect <<-EOF && diff --git a/- b/a/1 -- cgit 1.2.3-korg