[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 at arm.com> 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
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