Re: [PATCH v10 07/11] signal, x86: add SIGSYS info and make it synchronous.

[Will Drewry <wad@xxxxxxxxxxxx>]

On Thu, Feb 23, 2012 at 11:38 AM, Roland McGrath <mcgrathr@xxxxxxxxxx> wrote:
> On Wed, Feb 22, 2012 at 5:06 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:
>> I meant whether or not a signal can be blocked/caught and the fact that
>> the signal exists at all.
>>
>> Now I guess we could have "blockable" and "unblockable" SIGSYS, but that
>> would seem to have its own set of issues...
>
> Oh.  I certainly don't think we should ever add any new signals to the set
> that cannot be caught, blocked, or ignored.  That has been just SIGKILL and
> SIGSTOP since 4.2BSD, which first introduced the modern concept of blocking
> signals.  There are lots of reasons not to change that, which I won't go
> into unless someone really wants me to.
>
> However, I don't think there is anything really wrong with having certain
> cases that generate a signal and at the same time unblock it and reset it
> to SIG_DFL.  That's just an implementation detail of a policy of "dump core
> right now, no other option".  (Conversely, directly calling do_exit won't
> ever dump core, though it can be made to look signalesque to the parent and
> tracers.)
>
> For seccomp-filter, I personally don't see any problem with simply
> generating SIGSYS in the normal way (and aborting the syscall, of course).
> If someone wants to ensure that SIGSYS is never caught or blocked, they can
> just do that by having a filter that doesn't allow it to be caught or
> blocked (and of course make sure to reset its inherited state).  It is a
> bit tricky to cover all the ways, since it's not just sigaction and
> sigprocmask but also sigreturn, where the blocked signal set to be restored
> is in a slightly arcane location--but it ain't rocket science.
>
> But I don't really have any strong opinion about what seccomp-filter should
> do.  (Though it does seem worthwhile not to rule out the possibility of
> dumping core on a policy violation, since that will be useful for people to
> debug their code.)

Seems like there's an argument for another return code,
SECCOMP_RET_CORE, that resets/unblocks the SIGSYS handler since the
existing TRAP and KILL options seem to cover the other paths (signal
handler and do_exit).

It's a very small tweak if that'd be useful to include explicitly.

Thanks!
will
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