From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave P Martin 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 Message-ID: <20190523161256.GF2019@e103592.cambridge.arm.com> References: <20190523003916.20726-1-ebiederm@xmission.com> <20190523003916.20726-4-ebiederm@xmission.com> <20190523102101.GW28398@e103592.cambridge.arm.com> <87r28pgr3h.fsf@xmission.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Return-path: In-Reply-To: <87r28pgr3h.fsf@xmission.com> Content-Language: en-US Content-ID: Sender: linux-kernel-owner@vger.kernel.org To: "Eric W. Biederman" Cc: "linux-kernel@vger.kernel.org" , Linux Containers , Oleg Nesterov , "linux-arch@vger.kernel.org" , James Morse , Will Deacon List-Id: linux-arch.vger.kernel.org On Thu, May 23, 2019 at 03:53:06PM +0100, Eric W. Biederman wrote: > Dave Martin 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 mor= e > > 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 confid= ential and may also be privileged. If you are not the intended recipient, p= lease 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. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-eopbgr60076.outbound.protection.outlook.com ([40.107.6.76]:39480 "EHLO EUR04-DB3-obe.outbound.protection.outlook.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1730782AbfEWQND (ORCPT ); Thu, 23 May 2019 12:13:03 -0400 From: Dave P Martin 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 Message-ID: <20190523161256.GF2019@e103592.cambridge.arm.com> References: <20190523003916.20726-1-ebiederm@xmission.com> <20190523003916.20726-4-ebiederm@xmission.com> <20190523102101.GW28398@e103592.cambridge.arm.com> <87r28pgr3h.fsf@xmission.com> In-Reply-To: <87r28pgr3h.fsf@xmission.com> Content-Language: en-US Content-Type: text/plain; charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Eric W. Biederman" Cc: "linux-kernel@vger.kernel.org" , Linux Containers , Oleg Nesterov , "linux-arch@vger.kernel.org" , James Morse , Will Deacon Message-ID: <20190523161259.Iw7q51dUEB0VlXXv9sDXar_IwUkMMCSCHKoZeQwshsk@z> On Thu, May 23, 2019 at 03:53:06PM +0100, Eric W. Biederman wrote: > Dave Martin 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 mor= e > > 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 confid= ential and may also be privileged. If you are not the intended recipient, p= lease 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.