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Date:   Tue, 27 Nov 2018 06:48:49 +0300
From:   "Dmitry V. Levin" <ldv@altlinux.org>
To:     Elvira Khabirova <lineprinter@altlinux.org>,
        Oleg Nesterov <oleg@redhat.com>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Eugene Syromyatnikov <esyr@redhat.com>,
        linux-kernel@vger.kernel.org, strace-devel@lists.strace.io,
        linux-api@vger.kernel.org
Subject: Re: [PATCH RESEND v3 1/3] ptrace: pass type of a syscall-stop in
 ptrace_message
Message-ID: <20181127034849.GA4319@altlinux.org>
References: <20181125022150.46258a20@akathisia>
 <20181125022227.55e8caf7@akathisia>
 <20181126145643.GD1660@redhat.com>
 <20181127035357.0b381323@akathisia>
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On Tue, Nov 27, 2018 at 03:53:57AM +0100, Elvira Khabirova wrote:
> On Mon, 26 Nov 2018 15:56:43 +0100, Oleg Nesterov wrote:
> > On 11/25, Elvira Khabirova wrote:
> > >
> > > + * These values are stored in task->ptrace_message by tracehook_repo=
rt_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 =20
> >=20
> > Stupid question, why not
> >=20
> > 	#define PTRACE_EVENT_SYSCALL_ENTRY	8
> > 	#define PTRACE_EVENT_SYSCALL_EXIT	9
> >=20
> > right after other PTRACE_EVENT_* constants?
>=20
> I thought about adding new events for syscall {entry,exit}.
> For tracers, using new events means setting new options and checking
> for new values after waitpid(). They will also have to switch from using
> PTRACE_SYSCALL to PTRACE_CONT.
> Right now (with this version of the patch) tracers can use
> PTRACE_GETEVENTMSG without doing any additional configuration.
> More importantly, adding these events would require much more complex
> modifications of kernel code than this patch does.

To be honest, we don't see any way of introducing PTRACE_EVENT_* support
in PTRACE_GET_SYSCALL_INFO without adding new fields to struct task_struct.

> The only benefit I see from adding these events instead of letting
> syscall-stops put a value in ptrace_message is an ability to subscribe
> to syscall entries, but not to exits, and vice-versa, and I don't think
> it is worth it.


--=20
ldv

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