[PATCH] commit: check result of resolve_ref_unsafe

[Andrey Okoshkin <a.okoshkin@samsung.com>]

Add check of the resolved HEAD reference while printing of a commit summary.
resolve_ref_unsafe() may return NULL pointer if underlying calls of lstat() or
open() fail in files_read_raw_ref().
Such situation can be caused by race: file becomes inaccessible to this moment.

Signed-off-by: Andrey Okoshkin <a.okoshkin@samsung.com>
---
Hello,
I've injected a fault to git binary with the internal tool for fault tolerance
evaluation.

lstat() or open() calls return '-1', errno is set to 'EACCES':
#0 0x6559a4 in files_read_raw_ref refs/files-backend.c:686
#1 0x642816 in refs_read_raw_ref /home/tesla/devel/repos/git/refs.c:1392
#2 0x642a69 in refs_resolve_ref_unsafe /home/tesla/devel/repos/git/refs.c:1431
#3 0x6443ce in resolve_ref_unsafe /home/tesla/devel/repos/git/refs.c:1483
#4 0x44822e in print_summary builtin/commit.c:1485
#5 0x44822e in cmd_commit builtin/commit.c:1817
#6 0x4084f5 in run_builtin /home/tesla/devel/repos/git/git.c:342
#7 0x4084f5 in handle_builtin /home/tesla/devel/repos/git/git.c:550
#8 0x40997b in run_argv /home/tesla/devel/repos/git/git.c:602
#9 0x40997b in cmd_main /home/tesla/devel/repos/git/git.c:679
#10 0x408087 in main /home/tesla/devel/repos/git/common-main.c:43

As a result git crashes silently with SIGSEGV at 'strcmp(head, "HEAD")':
#0 0x447c16 in print_summary builtin/commit.c:1486
#1 0x447c16 in cmd_commit builtin/commit.c:1817
#2 0x4084f5 in run_builtin /home/tesla/devel/repos/git/git.c:342
#3 0x4084f5 in handle_builtin /home/tesla/devel/repos/git/git.c:550
#4 0x40997b in run_argv /home/tesla/devel/repos/git/git.c:602
#5 0x40997b in cmd_main /home/tesla/devel/repos/git/git.c:679
#6 0x408087 in main /home/tesla/devel/repos/git/common-main.c:43

It seems that in a real life it's very difficult to reproduce such behaviour
because the readability of '.git' directory is checked before. But still the
NULL pointer result returned by resolve_ref_unsafe() is not checked anyhow.
That's why I'm not sure whether it's a bug or not.

Best regards,
Andrey

 builtin/commit.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/builtin/commit.c b/builtin/commit.c
index 1a0da71a4..71a58dea3 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1483,6 +1483,8 @@ static void print_summary(const char *prefix, const struct object_id *oid,
 	diff_setup_done(&rev.diffopt);
 
 	head = resolve_ref_unsafe("HEAD", 0, junk_oid.hash, NULL);
+	if (!head)
+		BUG("unable to resolve HEAD reference");
 	if (!strcmp(head, "HEAD"))
 		head = _("detached HEAD");
 	else
-- 
2.14.2