Re: [PATCH v4 1/2] ptrace: save the type of syscall-stop in ptrace_message

From: "Dmitry V. Levin" <ldv@altlinux.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Elvira Khabirova <lineprinter@altlinux.org>,
	Eugene Syromyatnikov <esyr@redhat.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-kernel@vger.kernel.org, Andy Lutomirski <luto@kernel.org>,
	linux-api@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
	strace-devel@lists.strace.io
Subject: Re: [PATCH v4 1/2] ptrace: save the type of syscall-stop in ptrace_message
Date: Wed, 28 Nov 2018 17:05:34 +0300	[thread overview]
Message-ID: <20181128140533.GF28206@altlinux.org> (raw)
In-Reply-To: <20181128134913.GC30395@redhat.com>

[-- Attachment #1: Type: text/plain, Size: 1358 bytes --]

On Wed, Nov 28, 2018 at 02:49:14PM +0100, Oleg Nesterov wrote:
> On 11/28, Dmitry V. Levin wrote:
> >
> > +/*
> > + * These values are stored in task->ptrace_message by tracehook_report_syscall_*
> > + * to describe current syscall-stop.
> > + *
> > + * Values for these constants are chosen so that they do not appear
> > + * in task->ptrace_message by other means.
> > + */
> > +#define PTRACE_EVENTMSG_SYSCALL_ENTRY	0x80000000U
> > +#define PTRACE_EVENTMSG_SYSCALL_EXIT	0x90000000U
> 
> Again, I do not really understand the comment... Why should we care about
> "do not appear in task->ptrace_message by other means" ?
> 
> 2/2 should detect ptrace_report_syscall() case correctly, so we can use any
> numbers, say, 1 and 2?
> 
> If debugger does PTRACE_GETEVENTMSG it should know how to interpet the value
> anyway after wait(status).

Given that without this patch the value returned by PTRACE_GETEVENTMSG
during syscall stop is undefined, we need two different ptrace_message
values that cannot be set by other ptrace events to enable reliable
identification of syscall-enter-stop and syscall-exit-stop in userspace:
if we make PTRACE_GETEVENTMSG return 0 or any other value routinely set by
other ptrace events, it would be hard for userspace to find out whether
the kernel implements new semantics or not.

-- 
ldv

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 801 bytes --]