selftests: arm64: Check fread return value in exec_target

Fix -Wunused-result warning generated when compiled with gcc 13.3.0,
by checking fread's return value and handling errors, preventing
potential failures when reading from stdin.

Fixes compiler warning:
warning: ignoring return value of 'fread' declared with attribute
'warn_unused_result' [-Wunused-result]

Fixes: 806a15b ("kselftests/arm64: add PAuth test for whether exec() changes keys")

Signed-off-by: Bala-Vignesh-Reddy <reddybalavignesh9979@gmail.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>

Author: Bala-Vignesh-Reddy

tools/testing/selftests/arm64/pauth/exec_target.c

@@ -13,7 +13,12 @@ int main(void)
 	unsigned long hwcaps;
 	size_t val;
 
-	fread(&val, sizeof(size_t), 1, stdin);
+	size_t size = fread(&val, sizeof(size_t), 1, stdin);
+
+	if (size != 1) {
+		fprintf(stderr, "Could not read input from stdin\n");
+		return EXIT_FAILURE;
+	}
 
 	/* don't try to execute illegal (unimplemented) instructions) caller
 	 * should have checked this and keep worker simple