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    SubjectRe: [PATCH v6 1/2] ptrace,syscall_user_dispatch: Implement Syscall User Dispatch Suspension
    On Thu, Jan 26, 2023 at 7:07 AM Oleg Nesterov <oleg@redhat.com> wrote:
    >
    > On 01/25, Andrei Vagin wrote:
    > >
    > > On Wed, Jan 25, 2023 at 4:30 PM Oleg Nesterov <oleg@redhat.com> wrote:
    > > >
    > > > On 01/24, Gregory Price wrote:
    > > > >
    > > > > Adds PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH to ptrace options, and
    > > > > modify Syscall User Dispatch to suspend interception when enabled.
    > > > >
    > > > > This is modeled after the SUSPEND_SECCOMP feature, which suspends
    > > > > SECCOMP interposition. Without doing this, software like CRIU will
    > > > > inject system calls into a process and be intercepted by Syscall
    > > > > User Dispatch, either causing a crash (due to blocked signals) or
    > > > > the delivery of those signals to a ptracer (not the intended behavior).
    > > >
    > > > Cough... Gregory, I am sorry ;)
    > > >
    > > > but can't we drop this patch to ?
    > > >
    > > > CRIU needs to do PTRACE_SET_SYSCALL_USER_DISPATCH_CONFIG and check
    > > > config->mode anyway as we discussed.
    > > >
    > > > Then it can simply set *config->selector = SYSCALL_DISPATCH_FILTER_ALLOW
    > > > with the same effect, no?
    > >
    > > Oleg,
    > >
    > > PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH is automatically cleared when
    > > a tracer detaches. It is critical when tracers detach due to unexpected
    > > reasons
    >
    > 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.

    We want to have this functionality for most cases. I don't expect that
    the syscall user dispatch
    is used by many applications, so I don't strongly insist on
    PTRACE_O_SUSPEND_SYSCALL_USER_DISPATCH. In addition, if we know a user dispatch
    memory region, it can be enough to inject our code out of this region
    without disabling SUD.

    Thanks,
    Andrei

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