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

[Oleg Nesterov <oleg@redhat.com>]

On 01/26, Andrei Vagin wrote:
>
> On Thu, Jan 26, 2023 at 7:07 AM Oleg Nesterov <oleg@redhat.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.