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

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Date:   Thu, 22 Nov 2018 02:56:34 +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>,
        Sasha Levin <sasha.levin@...cle.com>,
        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 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.

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.

> >         __u8 __pad0[7];
> >         union {
> >                 struct {
> >                         __s32 nr;
> 
> __u64 please.  Syscall numbers are, as a practical matter, 64 bits.
> Admittedly, the actual effects of setting the high bits are unclear,
> and seccomp has issues with it, but let's not perpetuate the problem.

I agree.  Although the implementation uses syscall_get_nr()
which returns int, this could potentially be fixed in the future.

> >                         __u32 arch;
> >                         __u64 instruction_pointer;
> >                         __u64 args[6];
> >                 } entry_info;
> >                 struct {
> >                         __s64 rval;
> >                         __u8 is_error;
> >                         __u8 __pad1[7];
> >                 } exit_info;
> >         };
> > };
> 
> Should seccomp events use entry_info or should they just literally
> supply seccomp_data?

It certainly can use entry_info.
I'd prefer to avoid using in uapi/linux/ptrace.h those types
that are defined in uapi/linux/seccomp.h.


-- 
ldv

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