linux-kernel - Re: [RFC PATCH v2] ptrace: add PTRACE_GET_SYSCALL

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Date:   Thu, 22 Nov 2018 22:15:04 +0300
From:   "Dmitry V. Levin" <ldv@...linux.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Elvira Khabirova <lineprinter@...linux.org>,
        Kees Cook <keescook@...omium.org>,
        Linux API <linux-api@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        strace-devel@...ts.strace.io
Subject: Re: [RFC PATCH v2] ptrace: add PTRACE_GET_SYSCALL_INFO request

On Thu, Nov 22, 2018 at 06:55:29AM -0800, Andy Lutomirski wrote:
> On Wed, Nov 21, 2018 at 3:56 PM Dmitry V. Levin wrote:
> > On Wed, Nov 21, 2018 at 02:56:57PM -0800, Andy Lutomirski wrote:
> > > Please cc linux-api@...r.kernel.org for future versions.
> > >
> > > On Wed, Nov 21, 2018 at 7:58 AM Elvira Khabirova wrote:
> > > >
> > > > struct ptrace_syscall_info {
> > > >         __u8 op; /* 0 for entry, 1 for exit */
> > >
> > > Can you add proper defines, like:
> > >
> > > #define PTRACE_SYSCALL_ENTRY 0
> > > #define PTRACE_SYSCALL_EXIT 1
> > > #define PTRACE_SYSCALL_SECCOMP 2
> > >
> > > and make seccomp work from the start?  I'd rather we don't merge an
> > > implementation that doesn't work for seccomp and then have to rework
> > > it later.
> >
> > What's the difference between PTRACE_EVENT_SECCOMP and syscall-entry-stop
> > with regards to PTRACE_GET_SYSCALL_INFO request?  At least they have the
> > same entry_info to return.
> 
> I'm not sure there's any material difference.

In that case we don't really need PTRACE_SYSCALL_SECCOMP: op field
describes the structure inside the union to use, not the ptrace stop.

> > As long as implementation (ab)uses ptrace_message to tell one kind of stop
> > from another, it can distinguish syscall-entry-stop and syscall-exit-stop
> > from each other and from many other kinds of stops, but it cannot
> > distinguish PTRACE_EVENT_SECCOMP from e.g. PTRACE_EVENT_EXIT.
> 
> Hmm.  PTRACE_GET_SYSCALL_INFO should fail for PTRACE_EVENT_EXIT, I think.

Unless we can change PTRACE_EVENT_SECCOMP to set some higher bits of
ptrace_message (beyond SECCOMP_RET_DATA) which is very unlikely because
it would qualify as an ABI change, this would require an additional field
in struct task_struct because ptrace_message wouldn't be enough
to distinguish PTRACE_EVENT_SECCOMP from PTRACE_EVENT_EXIT.


-- 
ldv

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