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 <[email protected]>
---
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);
}
static FILE *seek_to_smaps_entry(unsigned long addr)
---
base-commit: 518e5b794c06c0f0eb40df3e202274a66202c137
change-id: 20260923-selftests-mm-mlock2-fix-fee59d31b124
Best regards,
--
Park Tae-sun <[email protected]>
