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

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Date:   Thu, 26 Jan 2023 19:30:19 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     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>,
        Gregory Price <gregory.price@...verge.com>,
        Mike Rapoport <rppt@...nel.org>
Subject: Re: [PATCH v6 1/2] ptrace,syscall_user_dispatch: Implement Syscall
 User Dispatch Suspension

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.