name-rev: avoid cutoff timestamp underflow

szeder · gitster · commit 2e09c01232b1 · 2019-09-28T13:36:04.000+09:00
When 'git name-rev' is invoked with commit-ish parameters, it tries to save some work, and doesn't visit commits older than the committer date of the oldest given commit minus a one day worth of slop. Since our 'timestamp_t' is an unsigned type, this leads to a timestamp underflow when the committer date of the oldest given commit is within a day of the UNIX epoch. As a result the cutoff timestamp ends up far-far in the future, and 'git name-rev' doesn't visit any commits, and names each given commit as 'undefined'. Check whether subtracting the slop from the oldest committer date would lead to an underflow, and use no cutoff in that case. We don't have a TIME_MIN constant, dddbad7 (timestamp_t: a new data type for timestamps, 2017-04-26) didn't add one, so do it now. Note that the type of the cutoff timestamp variable used to be signed before 5589e87 (name-rev: change a "long" variable to timestamp_t, 2017-05-20). The behavior was still the same even back then, but the underflow didn't happen when substracting the slop from the oldest committer date, but when comparing the signed cutoff timestamp with unsigned committer dates in name_rev(). IOW, this underflow bug is as old as 'git name-rev' itself. Helped-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/name-rev.c b/builtin/name-rev.c
@@ -9,7 +9,11 @@
 #include "sha1-lookup.h"
 #include "commit-slab.h"
 
-#define CUTOFF_DATE_SLOP 86400 /* one day */
+/*
+ * One day.  See the 'name a rev shortly after epoch' test in t6120 when
+ * changing this value
+ */
+#define CUTOFF_DATE_SLOP 86400
 
 typedef struct rev_name {
 	const char *tip_name;
@@ -481,8 +485,13 @@ int cmd_name_rev(int argc, const char **argv, const char *prefix)
 		add_object_array(object, *argv, &revs);
 	}
 
-	if (cutoff)
-		cutoff = cutoff - CUTOFF_DATE_SLOP;
+	if (cutoff) {
+		/* check for undeflow */
+		if (cutoff > TIME_MIN + CUTOFF_DATE_SLOP)
+			cutoff = cutoff - CUTOFF_DATE_SLOP;
+		else
+			cutoff = TIME_MIN;
+	}
 	for_each_ref(name_ref, &data);
 
 	if (transform_stdin) {
diff --git a/git-compat-util.h b/git-compat-util.h
@@ -344,6 +344,7 @@ typedef uintmax_t timestamp_t;
 #define PRItime PRIuMAX
 #define parse_timestamp strtoumax
 #define TIME_MAX UINTMAX_MAX
+#define TIME_MIN 0
 
 #ifndef PATH_SEP
 #define PATH_SEP ':'
diff --git a/t/t6120-describe.sh b/t/t6120-describe.sh
@@ -424,4 +424,19 @@ test_expect_success 'describe complains about missing object' '
 	test_must_fail git describe $ZERO_OID
 '
 
+test_expect_success 'name-rev a rev shortly after epoch' '
+	test_when_finished "git checkout master" &&
+
+	git checkout --orphan no-timestamp-underflow &&
+	# Any date closer to epoch than the CUTOFF_DATE_SLOP constant
+	# in builtin/name-rev.c.
+	GIT_COMMITTER_DATE="@1234 +0000" \
+	git commit -m "committer date shortly after epoch" &&
+	old_commit_oid=$(git rev-parse HEAD) &&
+
+	echo "$old_commit_oid no-timestamp-underflow" >expect &&
+	git name-rev $old_commit_oid >actual &&
+	test_cmp expect actual
+'
+
 test_done
