Re: siginfo pid not populated from ptrace?

Eric W. Biederman
2018-11-12T19:22:37Z

Tycho Andersen <tycho@tycho.ws> writes:

> On Mon, Nov 12, 2018 at 12:30:25PM -0600, Eric W. Biederman wrote:
>> Tycho Andersen <tycho@tycho.ws> writes:
>>
>> > Hi Oleg,
>> >
>> > I've been running some tests on my seccomp series, and in one of the
>> > tests on v4.20-rc2, I noticed,
>> >
>> > [ RUN ] global.syscall_restart
>> > seccomp_bpf.c:2784:global.syscall_restart:Expected getpid() (1492) == info._sifields._kill.si_pid (0)
>> > global.syscall_restart: Test failed at step #22
>> >
>> > which seems unrelated to my series (the kernel was stock v4.20 with my
>> > patches on top).
>> >
>> > I've been running a lot of tests, and only seen this once, so it seems
>> > like a fairly rare race. I tried to look through the code but didn't
>> > see anything obvious. Thoughts?
>>
>> My guess would be pid namespaces, or stopping for a signal other than
>> SIGSTOP.
>>
>> If you can get this to reproduce at all it would be interesting to see
>> si_signo and si_code. So that we can see just which signal is in info,
>> and how it should be decoded.
>
> Sure, here's what I see,
>
> seccomp_bpf.c:2784:global.syscall_restart:Expected getpid() (2195) == info._sifields._kill.si_pid (0)
> seccomp_bpf.c:2785:global.syscall_restart:si_signo: 19
> seccomp_bpf.c:2786:global.syscall_restart:si_code: 0

That is SI_USER and SIGSTOP. So as expected.

>> I see this test at line 2736 in 4.20-rc1 so there are almost 50 lines of
>> change in your version of seccomp_bpf.c. So I hope I am reading the
>> proper test.
>
> Yes, sorry, that's additional test stuff from my user trap series. I
> haven't manage to reproduce it on stock v4.20-rc2, unfortunately. It
> could be that this is some memory corruption introduced by my series,
> but I'm running these tests with KASAN so hopefully it would complain?

I don't have a clue what KASAN would do. I think it is mostly concerned
with unitialized memory, so this might be slipping through the cracks
somewhere.

It can be easy to mess up siginfo. That is part of the reason I just
reworked things to use helpers most of the time when touching siginfo.

Eric