selftests/nolibc: prevent out of bounds access in expect_vfprintf

[ Upstream commit 9c5e490 ]

If read() fails and returns -1 (or returns garbage for some other
reason) buf would be accessed out of bounds.
Only use the return value of read() after it has been validated.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Sasha Levin <sashal@kernel.org>

tools/testing/selftests/nolibc/nolibc-test.c

@@ -769,7 +769,6 @@ static int expect_vfprintf(int llen, size_t c, const char *expected, const char
 	lseek(fd, 0, SEEK_SET);
 
 	r = read(fd, buf, sizeof(buf) - 1);
-	buf[r] = '\0';
 
 	fclose(memfile);
 
@@ -779,6 +778,7 @@ static int expect_vfprintf(int llen, size_t c, const char *expected, const char
 		return 1;
 	}
 
+	buf[r] = '\0';
 	llen += printf(" \"%s\" = \"%s\"", expected, buf);
 	ret = strncmp(expected, buf, c);