linux-kernel - Re: [REVIEW][PATCH 03/26] signal/arm64: Use force_sig not force_sig

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Date:   Thu, 23 May 2019 16:00:41 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Dave P Martin <Dave.Martin@....com>
Cc:     "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "linux-arch\@vger.kernel.org" <linux-arch@...r.kernel.org>,
        James Morse <James.Morse@....com>,
        Will Deacon <Will.Deacon@....com>
Subject: Re: [REVIEW][PATCH 03/26] signal/arm64: Use force_sig not force_sig_fault for SIGKILL

Dave P Martin <Dave.Martin@....com> writes:

> On Thu, May 23, 2019 at 03:53:06PM +0100, Eric W. Biederman wrote:
>> Dave Martin <Dave.Martin@....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).

Done in my v2 version of this patch.

Eric