linux-kernel - Re: [PATCH v6 1/2] ptrace,syscall_user_dispatch: Implement Syscall User Dispatch Suspension

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Message-ID: <Y9LKifpedUSMkHZE@memverge.com>
Date:   Thu, 26 Jan 2023 13:46:33 -0500
From:   Gregory Price <gregory.price@...verge.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andrei Vagin <avagin@...il.com>,
        Gregory Price <gourry.memverge@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH v6 1/2] ptrace,syscall_user_dispatch: Implement Syscall
 User Dispatch Suspension

On Thu, Jan 26, 2023 at 07:30:19PM +0100, Oleg Nesterov wrote:
> On 01/26, Andrei Vagin wrote:
> >
> > On Thu, Jan 26, 2023 at 7:07 AM Oleg Nesterov <oleg@...hat.com> wrote:
> > >
> > > IIUC, PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH is needed to run the injected
> > > code, and this also needs to change the state of the traced process. If
> > > the tracer (CRIU) dies while the tracee runs this code, I guess the tracee
> > > will have other problems?
> >
> > Our injected code can reheal itself if something goes wrong. The hack
> > here is that we inject
> > the code with a signal frame and it calls rt_segreturn to resume the process.
> 
> What will happen if CRIU dies and clears ->ptrace right before
> syscall_user_dispatch() checks PT_SUSPEND_SYSCALL_USER_DISPATCH ?
> 
> How the tracee will react to SIGSYS with unexpected .si_syscall ?
> 
> > I don't expect that
> > the syscall user dispatch
> > is used by many applications,
> 
> Agreed, so the case when CRIU will need to do the additional
> PTRACE_SET_SYSCALL_USER_DISPATCH_CONFIG twice to disable and then re-enable
> syscall_user_dispatch is unlikely.
> 
> > so I don't strongly insist on
> > PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH.
> 
> I too won't argue too much. but so far I do not feel there is enough
> justification for this feature ...
> 
> Oleg.
> 

I'm not married to the idea, just want to make sure I have the tools
needed to make checkpoint work.  The option seems like the easiest way
given the exclusion area issue.

One idea is to overwrite a portion of the exclusion area, but this
obviously can increase the complexity of the checkpoint process.

Another idea is to disable/enable SUD via get/set, but this produces
potential detach issues.

@Andrei i'm happy to take this to IRC or somewhere else out of band to
discuss the checkpoint specifics, if that would be preferable and you
are interested.