[PATCH v2] rev-parse: check lookup'ed commit references for NULL

[Elijah Newren <newren@gmail.com>]

Commits 2122f8b963d4 ("rev-parse: Add support for the ^! and ^@ syntax",
2008-07-26) and 3dd4e7320d ("Teach rev-parse the ... syntax.", 2006-07-04)
taught rev-parse new syntax, and used lookup_commit_reference() as part of
their logic.  Neither usage checked the returned commit to see if it was
non-NULL before using it.  Check for NULL and ensure an appropriate error
is reported to the user.

Reported by Florian Weimer and Todd Zullinger.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Elijah Newren <newren@gmail.com>
---

I would have used a Reported-by tag for Florian and Todd, but looking at
the bugzilla.redhat.com bug report doesn't show me Florian's email
address.  I grepped through git logs and found two associated with that
name, but didn't know if they were still accurate, or were a different
Florian.  So I just went with the sentence instead.

 builtin/rev-parse.c          | 8 ++++++--
 t/t6101-rev-parse-parents.sh | 8 ++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/builtin/rev-parse.c b/builtin/rev-parse.c
index a1e680b5e9..a0a0ace38d 100644
--- a/builtin/rev-parse.c
+++ b/builtin/rev-parse.c
@@ -282,6 +282,10 @@ static int try_difference(const char *arg)
 			struct commit *a, *b;
 			a = lookup_commit_reference(&start_oid);
 			b = lookup_commit_reference(&end_oid);
+			if (!a || !b) {
+				*dotdot = '.';
+				return 0;
+			}
 			exclude = get_merge_bases(a, b);
 			while (exclude) {
 				struct commit *commit = pop_commit(&exclude);
@@ -328,12 +332,12 @@ static int try_parent_shorthands(const char *arg)
 		return 0;
 
 	*dotdot = 0;
-	if (get_oid_committish(arg, &oid)) {
+	if (get_oid_committish(arg, &oid) ||
+	    !(commit = lookup_commit_reference(&oid))) {
 		*dotdot = '^';
 		return 0;
 	}
 
-	commit = lookup_commit_reference(&oid);
 	if (exclude_parent &&
 	    exclude_parent > commit_list_count(commit->parents)) {
 		*dotdot = '^';
diff --git a/t/t6101-rev-parse-parents.sh b/t/t6101-rev-parse-parents.sh
index 8c617981a3..7683e4a114 100755
--- a/t/t6101-rev-parse-parents.sh
+++ b/t/t6101-rev-parse-parents.sh
@@ -214,4 +214,12 @@ test_expect_success 'rev-list merge^-1x (garbage after ^-1)' '
 	test_must_fail git rev-list merge^-1x
 '
 
+test_expect_success 'rev-parse $garbage^@ does not segfault' '
+	test_must_fail git rev-parse $EMPTY_TREE^@
+'
+
+test_expect_success 'rev-parse $garbage...$garbage does not segfault' '
+	test_must_fail git rev-parse $EMPTY_TREE...$EMPTY_BLOB
+'
+
 test_done
-- 
2.17.0.1.gda85003413