Re: [REVIEW][PATCH 03/26] signal/arm64: Use force_sig not force_sig_fault for SIGKILL

[Dave P Martin]

On Thu, May 23, 2019 at 03:53:06PM +0100, Eric W. Biederman wrote:
> Dave Martin <Dave.Martin@xxxxxxx> writes:
>
> > On Thu, May 23, 2019 at 01:38:53AM +0100, Eric W. Biederman wrote:
> >> It really only matters to debuggers but the SIGKILL does not have any
> >> si_codes that use the fault member of the siginfo union.  Correct this
> >> the simple way and call force_sig instead of force_sig_fault when the
> >> signal is SIGKILL.
> >
> > I haven't fully understood the context for this, but why does it matter
> > what's in siginfo for SIGKILL?  My understanding is that userspace
> > (including ptrace) never gets to see it anyway for the SIGKILL case.
>
> Yes.  In practice I think it would take tracing or something very
> exotic to notice anything going wrong because the task will be killed.
>
> > Here it feels like SIGKILL is logically a synchronous, thread-targeted
> > fault: we must ensure that no subsequent insn in current executes (just
> > like other fault signal).  In this case, I thought we fall back to
> > SIGKILL not because there is no fault, but because we failed to
> > properly diagnose or report the type of fault that occurred.
> >
> > So maybe handling it consistently with other faults signals makes
> > sense.  The fact that delivery of this signal destroys the process
> > before anyone can look at the resulting siginfo feels like a
> > side-effect rather than something obviously wrong.
> >
> > The siginfo is potentially useful diagnostic information, that we could
> > subsequently provide a means to access post-mortem.
> >
> > I just dived in on this single patch, so I may be missing something more
> > fundamental, or just being pedantic...
>
> Not really.  I was working on another cleanup and this usage of SIGKILL
> came up.
>
> A synchronous thread synchronous fault gets us as far as the forc_sig
> family of functions.  That only leaves the question of which union
> member in struct siginfo we are using.  The union members are _kill,
> _fault, _timer, _rt, _sigchld, _sigfault, _sigpoll, and _sigsys.
>
> As it has prove quite error prone for people to fill out struct siginfo
> in the past by hand, I have provided a couple of helper functions for
> the common cases that come up such as: force_sig_fault,
> force_sig_mceerr, force_sig_bnderr, force_sig_pkuerr.  Each of those
> helper functions takes the information needed to fill out the union
> member of struct siginfo that kind of fault corresponds to.
>
> For the SIGKILL case the only si_code I see being passed SI_KERNEL.
> The SI_KERNEL si_code corresponds to the _kill union member while
> force_sig_fault fills in fields for the _fault union member.
>
> Because of the mismatch of which union member SIGKILL should be using
> and the union member force_sig_fault applies alarm bells ring in my head
> when I read the current arm64 kernel code.  Somewhat doubly so because
> the other fields in passed to force_sig_fault appear to be somewhat
> random when SIGKILL is the signal.
>
> So I figured let's preserve the usage of SIGKILL as a synchronous
> exception.  That seems legitimate and other folks do that as well but
> let's use force_sig instead of force_sig_fault instead.  I don't know if
> userspace will notice but at the very least we won't be providing a bad
> example for other kernel code to follow and we won't wind up be making
> assumptions that are true today and false tomorrow when some
> implementation detail changes.
>
> For imformation on what signals and si_codes correspond to which
> union members you can look at siginfo_layout.  That function
> is the keeper of the magic decoder key.  Currently the only two
> si_codes defined for SIGKILL are SI_KERNEL and SI_USER both of which
> correspond to a _kill union member.

I see.  Assuming we cannot have a dummy internal si_code for this
special case (probably a bad idea), I think Will's suggestion of at
least pushing the special case handling down into
arm64_force_sig_fault() is probably a bit cleaner here, expecially
if other callers of that function may pass in SIGKILL (I haven't
looked though).

Cheers
---Dave
IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you.