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: 806a15b2545e ("kselftests/arm64: add PAuth test for whether exec() changes keys")

Signed-off-by: Bala-Vignesh-Reddy <[e-mail]>
Reviewed-by: Mark Brown <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

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

···
 	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