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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Shuah Khan , Brendan Jackman Subject: Re: [PATCH] selftests/mm: fix mlock2 errno handling and false PASS on ENOSYS To: Park Tae-sun References: <20260923-selftests-mm-mlock2-fix-v1-1-750b627854c6@dgu.ac.kr> From: Usama Anjum Content-Language: en-US In-Reply-To: <20260923-selftests-mm-mlock2-fix-v1-1-750b627854c6@dgu.ac.kr> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: gmyykhowmsaumf1cepech8u3fssagdmg X-Rspam-User: X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 237D840002 X-HE-Tag: 1790239223-840208 X-HE-Meta: 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 CVpTws8V FRx3VzaOPxTkRU7DpRZ8n571qm6a4/DuAZ4uB/HRXAW1wXPzPqvj5dX2TRhUzF0w0KVKDyMxrIx9vnVLRCptXNwUZufn9vRMM6ns1C+zASN1+OoDSyvN0Vv5Q7pWFspR1wB4CdEzkUhMap2bdRcqAms7jy5ufkroi/2WY+zRQFuOQCCzIL1w0yzrxglKmJXG74TNzo0kCycpMKHFawd7nliunu4UM0esA3JHUZiLhnkHpwmZdlMFHvk/EghQ12v2seO+An6rpd+WK6xFyw6D8zZ599Sta9wSvlrU+qXT2CEk3N5s= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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 > --- > 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 > > static FILE *seek_to_smaps_entry(unsigned long addr) > > --- > base-commit: 518e5b794c06c0f0eb40df3e202274a66202c137 > change-id: 20260923-selftests-mm-mlock2-fix-fee59d31b124 > > Best regards,