On Sat, 2025-08-16 at 14:43 +0000, Wei Yang wrote:
Hi Wei,
> On Sat, Aug 16, 2025 at 09:31:11AM +0530, Aboorva Devarajan wrote:
> > In ksm_functional_tests, test_child_ksm() returned negative values to
> > indicate errors. However, when passed to exit(), these were interpreted
> > as large unsigned values (e.g, -2 became 254), leading to incorrect
> > handling in the parent process. As a result, some tests appeared to be
> > skipped or silently failed.
>
> This is because "the least significant 8 bits" is returned to parent, right?
>
>
Yes, that's right. As per the WEXITSTATUS(wstatus) manual:
WEXITSTATUS: returns the exit status of the child. This consists of the
least significant 8 bits of the status argument that the child
specified in a call to exit(3) or _exit(2) or as the argument for a
return statement in main(). This macro should only be employed if
WIFEXITED returned true.
Since only the least significant 8 bits are preserved, negative return
values can appear as large unsigned codes, so using small positive exit
codes ensures the parent interprets the error code correctly.
> > This patch changes test_child_ksm() to return positive error codes (1, 2,
> > 3) and updates test_child_ksm_err() to interpret them correctly.
> > Additionally, test_prctl_fork_exec() now uses exit(4) after a failed
> > execv() to clearly signal exec failures. This ensures the parent
> > accurately detects and reports child process failures.
> >
> > --------------
> > Before patch:
> > --------------
> > - [RUN] test_unmerge
> > ok 1 Pages were unmerged
> > ...
> > - [RUN] test_prctl_fork
> > - No pages got merged
> > - [RUN] test_prctl_fork_exec
> > ok 7 PR_SET_MEMORY_MERGE value is inherited
> > ...
> > Bail out! 1 out of 8 tests failed
> > - Planned tests != run tests (9 != 8)
> > - Totals: pass:7 fail:1 xfail:0 xpass:0 skip:0 error:0
> >
> > --------------
> > After patch:
> > --------------
> > - [RUN] test_unmerge
> > ok 1 Pages were unmerged
> > ...
> > - [RUN] test_prctl_fork
> > - No pages got merged
> > not ok 7 Merge in child failed
> > - [RUN] test_prctl_fork_exec
> > ok 8 PR_SET_MEMORY_MERGE value is inherited
> > ...
> > Bail out! 2 out of 9 tests failed
> > - Totals: pass:7 fail:2 xfail:0 xpass:0 skip:0 error:0
> >
> > Fixes: 6c47de3be3a0 ("selftest/mm: ksm_functional_tests: extend test case
> > for ksm fork/exec")
> > Co-developed-by: Donet Tom <donet...@linux.ibm.com>
> > Signed-off-by: Donet Tom <donet...@linux.ibm.com>
> > Acked-by: David Hildenbrand <da...@redhat.com>
> > Signed-off-by: Aboorva Devarajan <aboor...@linux.ibm.com>
>
> If so:
>
> Reviewed-by: Wei Yang <richard.weiy...@gmail.com>
>
> Thanks, I am afraid to make the same mistake if you don't point out.
Thanks,
Aboorva
