linux-kernel - Re: `(T) stopped' preservation after _exit() [Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after PTRACE

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Date:	Sun, 20 Feb 2011 22:19:13 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jan Kratochvil <jan.kratochvil@...hat.com>
Cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	Tejun Heo <tj@...nel.org>, Roland McGrath <roland@...hat.com>,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: `(T) stopped' preservation after _exit()  [Re: [PATCH 1/1]
	ptrace: make sure do_wait() won't hang after PTRACE_ATTACH]

On 02/20, Jan Kratochvil wrote:
>
> On Sun, 20 Feb 2011 21:38:19 +0100, Oleg Nesterov wrote:
> >
> > This doesn't explain why the kernel should restore TASK_STOPPED.
> > Suppose the tracee creates the file when it was run under debugger,
> > should the kernel remove this file after detach?
>
> No, opening such file is like writing data into a file.

OK, let it be writing data into a file. But in this case the kernel
shouldn't truncate the file after detach or if the tracer crashes ;)

> > And, if SIGCONT comes in between, the tracee is no longer stopped. It needs
> > another SIGSTOP, gdb can do this via ptrace(DETACH, SIGSTOP).
>
> It cannot if it crashes in between.

Yes,

> It would be OK if it can do so right
> after the inferior call.  Which I realize now it can, after the inferior call
> returns (wait->SIGTRAP) GDB can do PTRACE_CONT(SIGSTOP), wait->SIGSTOP and now
> it can do PTRACE_GETREGS etc. while after _exit() it will be like after
> PTRACE_DETACH(0) and the debuggee still remains `(T) stopped', doesn't it?

Yes. Or gdb can just send SIGSTOP to the tracee.

Modulo other bugs we have, but these bugs should be fixed anyway.

For example, once again, PTRACE_DETACH/PTRACE_CONT can ignore SIGXXX.
I never knew if it was designed this way or this should be fixed, but
there is one particular case which looks like the oversight to me: If
the tracee reports SIGTRAP after it steps into the signal handler, then
SIGXXX is ignored after PTRACE_CONT/DETACH (and this btw affects gdb).

Oleg.

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