linux-kernel - Re: [PATCH v6 6/9] kernel: entry: Support Syscall User Dispatch for common syscall entry

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Date:   Fri, 11 Sep 2020 11:46:34 +0200
From:   peterz@...radead.org
To:     Gabriel Krisman Bertazi <krisman@...labora.com>
Cc:     luto@...nel.org, tglx@...utronix.de, keescook@...omium.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, willy@...radead.org,
        linux-kselftest@...r.kernel.org, shuah@...nel.org,
        kernel@...labora.com
Subject: Re: [PATCH v6 6/9] kernel: entry: Support Syscall User Dispatch for
 common syscall entry

On Fri, Sep 04, 2020 at 04:31:44PM -0400, Gabriel Krisman Bertazi wrote:
> Syscall User Dispatch (SUD) must take precedence over seccomp, since the
> use case is emulation (it can be invoked with a different ABI) such that
> seccomp filtering by syscall number doesn't make sense in the first
> place.  In addition, either the syscall is dispatched back to userspace,
> in which case there is no resource for seccomp to protect, or the
> syscall will be executed, and seccomp will execute next.
> 
> Regarding ptrace, I experimented with before and after, and while the
> same ABI argument applies, I felt it was easier to debug if I let ptrace
> happen for syscalls that are dispatched back to userspace.  In addition,
> doing it after ptrace makes the code in syscall_exit_work slightly
> simpler, since it doesn't require special handling for this feature.

I think I'm with Andy that this should be before ptrace(). ptrace()
users will attempt to interpret things like they're regular syscalls,
and that's definitely not the case.