diff --git a/test-sha1.c b/test-sha1.c
index 2efc315ee3..78d7e983a7 100644
--- a/test-sha1.c
+++ b/test-sha1.c
@@ -4,21 +4,44 @@ int main(int ac, char **av)
 {
 	SHA_CTX ctx;
 	unsigned char sha1[20];
+	unsigned bufsz = 8192;
+	char *buffer;
+
+	if (ac == 2)
+		bufsz = strtoul(av[1], NULL, 10) * 1024 * 1024;
+
+	if (!bufsz)
+		bufsz = 8192;
+
+	while ((buffer = malloc(bufsz)) == NULL) {
+		fprintf(stderr, "bufsz %u is too big, halving...\n", bufsz);
+		bufsz /= 2;
+		if (bufsz < 1024)
+			die("OOPS");
+	}
 
 	SHA1_Init(&ctx);
 
 	while (1) {
-		ssize_t sz;
-		char buffer[8192];
-		sz = xread(0, buffer, sizeof(buffer));
-		if (sz == 0)
+		ssize_t sz, this_sz;
+		char *cp = buffer;
+		unsigned room = bufsz;
+		this_sz = 0;
+		while (room) {
+			sz = xread(0, cp, room);
+			if (sz == 0)
+				break;
+			if (sz < 0)
+				die("test-sha1: %s", strerror(errno));
+			this_sz += sz;
+			cp += sz;
+			room -= sz;
+		}
+		if (this_sz == 0)
 			break;
-		if (sz < 0)
-			die("test-sha1: %s", strerror(errno));
-		SHA1_Update(&ctx, buffer, sz);
+		SHA1_Update(&ctx, buffer, this_sz);
 	}
 	SHA1_Final(sha1, &ctx);
 	puts(sha1_to_hex(sha1));
 	exit(0);
 }
-
diff --git a/test-sha1.sh b/test-sha1.sh
index 01bbb5782e..640856af5a 100755
--- a/test-sha1.sh
+++ b/test-sha1.sh
@@ -11,7 +11,7 @@ do
 			test -z "$pfx" || echo "$pfx"
 			dd if=/dev/zero bs=1048576 count=$cnt 2>/dev/null |
 			tr '[\0]' '[g]'
-		} | ./test-sha1
+		} | ./test-sha1 $cnt
 	`
 	if test "$expect" = "$actual"
 	then