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From: Dave P Martin <Dave.Martin@arm.com>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Oleg Nesterov <oleg@redhat.com>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	James Morse <James.Morse@arm.com>,
	Will Deacon <Will.Deacon@arm.com>
Subject: Re: [REVIEW][PATCH 03/26] signal/arm64: Use force_sig not force_sig_fault for SIGKILL
Date: Thu, 23 May 2019 16:12:59 +0000	[thread overview]
Message-ID: <20190523161256.GF2019@e103592.cambridge.arm.com> (raw)
In-Reply-To: <87r28pgr3h.fsf@xmission.com>

On Thu, May 23, 2019 at 03:53:06PM +0100, Eric W. Biederman wrote:
> Dave Martin <Dave.Martin@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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  reply	other threads:[~2019-05-23 16:13 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-23  0:38 [REVIEW][PATCH 00/26] signal: Remove task argument from force_sig_info Eric W. Biederman
2019-05-23  0:38 ` [REVIEW][PATCH 01/26] signal: Correct namespace fixups of si_pid and si_uid Eric W. Biederman
     [not found]   ` <20190529131503.F2AC221871@mail.kernel.org>
2019-05-29 15:18     ` Eric W. Biederman
2019-05-23  0:38 ` [REVIEW][PATCH 02/26] signal/ptrace: Simplify and fix PTRACE_KILL Eric W. Biederman
2019-05-29 14:35   ` Eric W. Biederman
2019-05-23  0:38 ` [REVIEW][PATCH 03/26] signal/arm64: Use force_sig not force_sig_fault for SIGKILL Eric W. Biederman
2019-05-23 10:17   ` Will Deacon
2019-05-23 14:59     ` Eric W. Biederman
2019-05-23 16:11     ` [REVIEW][PATCHv2 " Eric W. Biederman
2019-05-23 16:15       ` Will Deacon
2019-05-23 20:59         ` Eric W. Biederman
2019-05-24 10:00           ` Will Deacon
2019-05-24 22:36             ` Eric W. Biederman
2019-05-29 15:12               ` Will Deacon
2019-05-29 15:34                 ` Eric W. Biederman
2019-05-23 10:21   ` [REVIEW][PATCH " Dave Martin
2019-05-23 14:53     ` Eric W. Biederman
2019-05-23 16:12       ` Dave P Martin [this message]
2019-05-23 21:00         ` Eric W. Biederman
2019-05-23  0:38 ` [REVIEW][PATCH 04/26] signal/drbd: Use send_sig not force_sig Eric W. Biederman
2019-05-23  0:38 ` [REVIEW][PATCH 05/26] signal/bpfilter: Fix bpfilter_kernl to use " Eric W. Biederman
2019-05-23  0:38 ` [REVIEW][PATCH 06/26] signal/pid_namespace: Fix reboot_pid_ns " Eric W. Biederman
2019-05-23  0:38 ` [REVIEW][PATCH 07/26] signal/cifs: Fix cifs_put_tcp_session to call send_sig instead of force_sig Eric W. Biederman
2019-05-23  0:38 ` [REVIEW][PATCH 08/26] signal: Remove task parameter from force_sigsegv Eric W. Biederman
2019-05-23  0:38 ` [REVIEW][PATCH 09/26] signal: Remove task parameter from force_sig Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 10/26] signal: Remove task parameter from force_sig_mceerr Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 11/26] signal/x86: Remove task parameter from send_sigtrap Eric W. Biederman
2019-05-28 18:18   ` Thomas Gleixner
2019-05-23  0:39 ` [REVIEW][PATCH 12/26] signal/um: " Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 13/26] signal/sh: Remove tsk parameter from force_sig_info_fault Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 14/26] signal/riscv: Remove tsk parameter from do_trap Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 15/26] signal/nds32: Remove tsk parameter from send_sigtrap Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 16/26] signal/arm: Remove tsk parameter from ptrace_break Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 17/26] signal/arm: Remove tsk parameter from __do_user_fault Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 18/26] signal/unicore32: " Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 19/26] signal: Explicitly call force_sig_fault on current Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 20/26] signal: Use force_sig_fault_to_task for the two calls that don't deliver to current Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 21/26] signal: Remove the task parameter from force_sig_fault Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 22/26] signal: Properly set TRACE_SIGNAL_LOSE_INFO in __send_signal Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 23/26] signal: Move the computation of force into send_signal and correct it Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 24/26] signal: Generate the siginfo in force_sig Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 25/26] signal: Factor force_sig_info_to_task out of force_sig_info Eric W. Biederman
2019-05-23  0:39 ` [REVIEW][PATCH 26/26] signal: Remove the signal number and task parameters from force_sig_info Eric W. Biederman
2019-05-24 23:35 ` [REVIEW][PATCH 00/26] signal: Remove task argument " Eric W. Biederman
2019-05-29 15:37 ` Eric W. Biederman

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