Re: [PATCH] selftests/mm: fix mlock2 errno handling and false PASS on ENOSYS

[Usama Anjum <usama.anjum@arm.com>]

On 23/09/2026 10:26 am, Park Tae-sun wrote:
> 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
> 
> 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>
> ---
>  tools/testing/selftests/mm/mlock2-tests.c | 2 +-
>  tools/testing/selftests/mm/mlock2.h       | 8 +-------
>  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);
>  }

Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>

>  
>  static FILE *seek_to_smaps_entry(unsigned long addr)
> 
> ---
> base-commit: 518e5b794c06c0f0eb40df3e202274a66202c137
> change-id: 20260923-selftests-mm-mlock2-fix-fee59d31b124
> 
> Best regards,