selftests/vm: fix errno handling in mrelease_test

mrelease_test should return KSFT_SKIP when process_mrelease is not
defined, but due to a perror call consuming the errno, it returns
KSFT_FAIL.

This patch decides the exit code before calling perror.

[adam@wowsignal.io: fix remaining instances of errno mishandling]
  Link: https://lkml.kernel.org/r/20220706141602.10159-1-adam@wowsignal.io
Link: https://lkml.kernel.org/r/20220704173351.19595-1-adam@wowsignal.io
Fixes: 33776141 ("selftests: vm: add process_mrelease tests")
Signed-off-by: Adam Sindelar <adam@wowsignal.io>
Reviewed-by: David Vernet <void@manifault.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/vm/mrelease_test.c

@@ -62,19 +62,22 @@ static int alloc_noexit(unsigned long nr_pages, int pipefd)
 /* The process_mrelease calls in this test are expected to fail */
 static void run_negative_tests(int pidfd)
 {
+	int res;
 	/* Test invalid flags. Expect to fail with EINVAL error code. */
 	if (!syscall(__NR_process_mrelease, pidfd, (unsigned int)-1) ||
 			errno != EINVAL) {
+		res = (errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
 		perror("process_mrelease with wrong flags");
-		exit(errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
+		exit(res);
 	}
 	/*
 	 * Test reaping while process is alive with no pending SIGKILL.
 	 * Expect to fail with EINVAL error code.
 	 */
 	if (!syscall(__NR_process_mrelease, pidfd, 0) || errno != EINVAL) {
+		res = (errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
 		perror("process_mrelease on a live process");
-		exit(errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
+		exit(res);
 	}
 }
 
@@ -100,8 +103,9 @@ int main(void)
 
 	/* Test a wrong pidfd */
 	if (!syscall(__NR_process_mrelease, -1, 0) || errno != EBADF) {
+		res = (errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
 		perror("process_mrelease with wrong pidfd");
-		exit(errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
+		exit(res);
 	}
 
 	/* Start the test with 1MB child memory allocation */
@@ -156,8 +160,9 @@ int main(void)
 	run_negative_tests(pidfd);
 
 	if (kill(pid, SIGKILL)) {
+		res = (errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
 		perror("kill");
-		exit(errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
+		exit(res);
 	}
 
 	success = (syscall(__NR_process_mrelease, pidfd, 0) == 0);
@@ -172,9 +177,10 @@ int main(void)
 		if (errno == ESRCH) {
 			retry = (size <= MAX_SIZE_MB);
 		} else {
+			res = (errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
 			perror("process_mrelease");
 			waitpid(pid, NULL, 0);
-			exit(errno == ENOSYS ? KSFT_SKIP : KSFT_FAIL);
+			exit(res);
 		}
 	}