diff --git a/builtin/commit.c b/builtin/commit.c
index 276c4f2e7e..2de5f6cc64 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1525,12 +1525,10 @@ static int git_commit_config(const char *k, const char *v, void *cb)
 static int run_rewrite_hook(const unsigned char *oldsha1,
 			    const unsigned char *newsha1)
 {
-	/* oldsha1 SP newsha1 LF NUL */
-	static char buf[2*40 + 3];
 	struct child_process proc = CHILD_PROCESS_INIT;
 	const char *argv[3];
 	int code;
-	size_t n;
+	struct strbuf sb = STRBUF_INIT;
 
 	argv[0] = find_hook("post-rewrite");
 	if (!argv[0])
@@ -1546,11 +1544,11 @@ static int run_rewrite_hook(const unsigned char *oldsha1,
 	code = start_command(&proc);
 	if (code)
 		return code;
-	n = snprintf(buf, sizeof(buf), "%s %s\n",
-		     sha1_to_hex(oldsha1), sha1_to_hex(newsha1));
+	strbuf_addf(&sb, "%s %s\n", sha1_to_hex(oldsha1), sha1_to_hex(newsha1));
 	sigchain_push(SIGPIPE, SIG_IGN);
-	write_in_full(proc.in, buf, n);
+	write_in_full(proc.in, sb.buf, sb.len);
 	close(proc.in);
+	strbuf_release(&sb);
 	sigchain_pop(SIGPIPE);
 	return finish_command(&proc);
 }