linux-kernel - Re: [PATCH RESEND v3 1/3] ptrace: pass type of a syscall-stop in ptrace

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Date:   Tue, 27 Nov 2018 03:53:57 +0100
From:   Elvira Khabirova <lineprinter@...linux.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     rostedt@...dmis.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, ldv@...linux.org, esyr@...hat.com,
        luto@...nel.org, strace-devel@...ts.strace.io,
        linux-api@...r.kernel.org
Subject: Re: [PATCH RESEND v3 1/3] ptrace: pass type of a syscall-stop in
 ptrace_message

On Mon, 26 Nov 2018 15:56:43 +0100
Oleg Nesterov <oleg@...hat.com> wrote:

> On 11/25, Elvira Khabirova 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  
> 
> Stupid question, why not
> 
> 	#define PTRACE_EVENT_SYSCALL_ENTRY	8
> 	#define PTRACE_EVENT_SYSCALL_EXIT	9
> 
> right after other PTRACE_EVENT_* constants?

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.
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.