selftests/mm: fix mlock2 errno handling and false PASS on ENOSYS

While inspecting selftests/mm syscall wrappers, I noticed that mlock2_()
in mlock2.h handles the syscall return value differently from other
wrappers:

    int ret = syscall(__NR_mlock2, start, len, flags);
    if (ret) {
        errno = ret;
        return -1;
    }

Commit 1ddae9d67ee1 ("selftests/mm/mlock: print error on failure")
introduced this intending to make mlock2_() behave like libc by setting
errno and returning -1.  However, glibc syscall(2) already returns -1 on
failure and sets positive errno.  Assigning "errno = ret;" overwrites
errno with -1.

To verify this, mlock2 was disabled in the kernel (via sys_ni_syscall) to
return -ENOSYS. Testing revealed two interrelated defects:

1. In the unmodified test, mlock2_() clobbered errno to -1. The check
   "if (ret && errno == ENOSYS)" in main() was bypassed, resulting in an
   immediate crash in the first test:

       ~ # ./mlock2-tests
       TAP version 13
       1..15
       Bail out! mlock2(0): Unknown error -1
       # Planned tests != run tests (15 != 0)
       # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
       (exit code: 1 - FAIL)

2. After restoring mlock2_() to directly return syscall(), errno correctly
   retained ENOSYS (38), entering the ENOSYS check in main(). However, it
   then called ksft_finished():

       ~ # ./mlock2-tests
       TAP version 13
       # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0
       ~ # echo $?
       0

   Because ksft_set_plan() had not been called yet (ksft_plan == 0) and
   zero tests ran (ksft_pass == 0), ksft_finished() evaluated 0 == 0 as
   success and exited with KSFT_PASS (code 0) without any TAP skip header.

Fix both issues by:
1. Returning the syscall() result directly in mlock2_() so that errno is
   preserved.
2. Calling ksft_exit_skip() on ENOSYS so unsupported kernels report a TAP
   skip ("1..0 # SKIP ...") and exit with KSFT_SKIP (code 4).

Verification on the mlock2-disabled kernel:

    ~ # ./mlock2-tests
    TAP version 13
    1..0 # SKIP mlock2() syscall is not supported
    ~ # echo $?
    4

Re-enabling mlock2 in the kernel confirmed all 15 tests pass cleanly:

    ~ # ./mlock2-tests
    TAP version 13
    1..15
    ok 1 test_mlock_lock: Locked
    ...
    ok 15 test_mlockall_future_droppable: droppable memory not locked
    # Totals: pass:15 fail:0 xfail:0 xpass:0 skip:0 error:0
    ~ # echo $?
    0

Link: https://lore.kernel.org/20260923-selftests-mm-mlock2-fix-v1-1-750b627854c6@dgu.ac.kr
Fixes: 1ddae9d67ee1 ("selftests/mm/mlock: print error on failure")
Fixes: 65c89684896d ("selftests/mm: mlock2-tests: conform test to TAP format output")
Signed-off-by: Park Tae-sun <ts930@dgu.ac.kr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Gregory Price <gourry@gourry.net>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Brendan Jackman <brendan.jackman@linux.dev>

2 files changed, 2 insertions, 8 deletions

diff --git a/tools/testing/selftests/mm/mlock2-tests.c b/tools/testing/selftests/mm/mlock2-tests.c
index e16e288cc7c1..144b550813a6 100644
--- a/tools/testing/selftests/mm/mlock2-tests.c
+++ b/tools/testing/selftests/mm/mlock2-tests.c
@@ -502,7 +502,7 @@ int main(int argc, char **argv)
 
 	ret = mlock2_(map, size, MLOCK_ONFAULT);
 	if (ret && errno == ENOSYS)
-		ksft_finished();
+		ksft_exit_skip("mlock2() syscall is not supported\n");
 
 	munmap(map, size);
 
diff --git a/tools/testing/selftests/mm/mlock2.h b/tools/testing/selftests/mm/mlock2.h
index 81e77fa41901..4417eaa5cfb7 100644
--- a/tools/testing/selftests/mm/mlock2.h
+++ b/tools/testing/selftests/mm/mlock2.h
@@ -6,13 +6,7 @@
 
 static int mlock2_(void *start, size_t len, int flags)
 {
-	int ret = syscall(__NR_mlock2, start, len, flags);
-
-	if (ret) {
-		errno = ret;
-		return -1;
-	}
-	return 0;
+	return syscall(__NR_mlock2, start, len, flags);
 }
 
 static FILE *seek_to_smaps_entry(unsigned long addr)