Re: [PATCH] virtiofsd: Add `sigreturn` to the seccomp whitelist

[Christian Borntraeger <borntraeger@de.ibm.com>]

Am 29.11.22 um 10:42 schrieb Dr. David Alan Gilbert:
> * Marc Hartmayer (mhartmay@linux.ibm.com) wrote:
>> "Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:
>>
>>> * Marc Hartmayer (mhartmay@linux.ibm.com) wrote:
>>>> The virtiofsd currently crashes on s390x. This is because of a
>>>> `sigreturn` system call. See audit log below:
>>>>
>>>> type=SECCOMP msg=audit(1669382477.611:459): auid=4294967295 uid=0 gid=0 ses=4294967295 subj=system_u:system_r:virtd_t:s0-s0:c0.c1023 pid=6649 comm="virtiofsd" exe="/usr/libexec/virtiofsd" sig=31 arch=80000016 syscall=119 compat=0 ip=0x3fff15f748a code=0x80000000AUID="unset" UID="root" GID="root" ARCH=s390x SYSCALL=sigreturn
>>>
>>> I'm curious; doesn't that mean that some signal is being delivered and
>>> you're returning?  Which one?
>>
>> code=0x80000000 means that the seccomp action SECCOMP_RET_KILL_PROCESS
>> is taken => process is killed by a SIGSYS signal (31) [1].
>>
>> At least, that’s my understanding of this log message.
>>
>> [1] https://man7.org/linux/man-pages/man2/seccomp.2.html
> 
> But isn't that the fallout rather than the cause ? i.e. seccomp
> is sending a SIGSYS because the process used sigreturn, my question
> is why did the process call sigreturn in the first place - it must
> have received a signal to return from?

Good question. virtiofsd seems to prepare itself for

int fuse_set_signal_handlers(struct fuse_session *se)
{
     /*
      * If we used SIG_IGN instead of the do_nothing function,
      * then we would be unable to tell if we set SIG_IGN (and
      * thus should reset to SIG_DFL in fuse_remove_signal_handlers)
      * or if it was already set to SIG_IGN (and should be left
      * untouched.
      */
     if (set_one_signal_handler(SIGHUP, exit_handler, 0) == -1 ||
         set_one_signal_handler(SIGINT, exit_handler, 0) == -1 ||
         set_one_signal_handler(SIGTERM, exit_handler, 0) == -1 ||
         set_one_signal_handler(SIGPIPE, do_nothing, 0) == -1) {
         return -1;
     }



Given that rt_sigreturn was already on the seccomp list it seems
to be expected that those handlers are called.