linux-kernel - Re: [RFC/PATCH] Implement new PTRACE_EVENT_SYSCALL

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Date:	Sun, 19 Jan 2014 16:29:22 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sergio Durigan Junior <sergiodj@...hat.com>
Cc:	Pedro Alves <palves@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Roland McGrath <roland@...k.frob.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Tom Tromey <tromey@...hat.com>,
	Jan Kratochvil <jan.kratochvil@...hat.com>,
	Tejun Heo <tj@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC/PATCH] Implement new PTRACE_EVENT_SYSCALL_{ENTER,EXIT}

On 01/19, Sergio Durigan Junior wrote:
>
> On Friday, January 10 2014, Oleg Nesterov wrote:
>
> > So suppose that gdb does ptrace(PTRACE_SINGLESTEP) and the tracee
> > executes the "syscall" insn. What it should report?
> [...]
> > But what should syscall-exit do? Should it still report SIGSEGV as
> > it currently does, or should it report _SYSCALL_EXIT instead (if
> > PTRACE_O_SYSCALL_EXIT of course), or should it report both?
>
> Both only if _SYSCALL_EXIT is set.  Otherwise, stick to the current
> behavior, I guess.

OK, both. In which order? Probably _EXIT first. But this looks a bit
strange. Suppose that the tracee reports _EXIT, then debugger does
ptrace(PTRACE_CONT), should the tracee report SIGTRAP?

SIGTRAP before _EXIT looks a bit strange too... Single-step trap should
be reported after insn, but we are still in syscall.

So perhaps _EXIT should win and do not report the step?

> Isn't it what my current patch does, by the way?

I forgot how this patch looks so I can be easily wrong, but iirc no.
Note that tracehook_report_syscall_exit() doesn't even call
ptrace_report_syscall() if step == T.

Btw, if you send v2, please CC Michael Kerrisk <mtk.manpages@...il.com>.

Oleg.

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