linux-kernel - Re: Ptrace documentation, draft #1

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Message-ID: <20110516165352.GA18727@redhat.com>
Date:	Mon, 16 May 2011 18:53:52 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	jan.kratochvil@...hat.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	indan@....nu
Subject: Re: Ptrace documentation, draft #1

On 05/16, Tejun Heo wrote:
>
> On Mon, May 16, 2011 at 05:31:22PM +0200, Oleg Nesterov wrote:
> > > Kernel delivers an extra SIGTRAP to tracee after execve syscall
> > > returns. This is an ordinary signal (similar to one generated by kill
> > > -TRAP), not a special kind of ptrace-stop. If PTRACE_O_TRACEEXEC option
> > > is in effect, a PTRACE_EVENT_EXEC-stop is generated instead.
> > >
> > > ??? can this SIGTRAP be distinguished from "real" user-generated SIGTRAP
> > >     by looking at its siginfo?
> >
> > Afaics no. Well, except .si_pid shows that the signal was sent by the
> > tracing process to itself.
>
> If you do GETSIGINFO and look at si->si_code, user generated signals
> can't have non-zero value there

Hmm. The can? sys_kill() sets si_code = 0, but tkill() or queueinfo()
can pass any si_code < 0. Also, the kernel can generate the signal
with si_code > 0.

> so, if si->si_code contains SIGTRAP |
> PTRACE_EVENT_* << 8,

But in this case (without PT_TRACE_EXEC) the tracee simply sends SIGTRAP
to itself. It will be reported later like a normal signal.

> > Yes. Except syscall entry/exit. But in this case SET_SIGINFO doesn't work
> > to add more confusion ;)
>
> Yeah, I hate these signal injections.

Welcome to the club!

> Maybe we can remove these if
> SEIZED?

Heh... I am not sure.

> > > 	ptrace(PTRACE_cmd, pid, 0, sig);
> > > where cmd is CONT, DETACH, SYSCALL, SINGLESTEP, SYSEMU,
> > > SYSEMU_SINGLESTEP. If tracee is in signal-delivery-stop, sig is the
> > > signal to be injected. Otherwise, sig is ignored.
> >
> > There is another special case. If the tracee single-stepps into the
> > signal handler, it reports SIGTRAP as if it recieved this SIGNAL.
> > But ptrace(PTRACE, ..., sig) doesn't inject after that.
>
> Ditto as PTRACE_SYSCALL.

Hmm. What do you mean? tracehook_report_syscall_exit() can skip
ptrace_report_syscall() if step, but this is another story. And in this
case the tracee sends the real signal to itself, unlike
tracehook_signal_handler() which does ptrace_notify().

> I think resuming from signal delivery trap
> should be the only place where @data means signo.

Agreed, it would be more clean/simple/understandable.

> > >     Is signal injected if they were in signal-delivery-stop?
> >
> > Yes, The tracee resumes and handles the previously reported signal.
>
> Please note that this one really isn't an injection.  It's just given
> a chance to either suppress or alter a signal which is being
> delivered.  It's not creating a new one.

Yes, agreed, we do not create the new one.

Oleg.

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