linux-kernel - Re: [PATCH 08/14] ptrace: make do_signal_stop() use ptrace

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Date:	Sun, 28 Nov 2010 21:22:02 +0100
From:	Jan Kratochvil <jan.kratochvil@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, roland@...hat.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org
Subject: Re: [PATCH 08/14] ptrace: make do_signal_stop() use ptrace_stop() if
 the task is being ptraced

On Sun, 28 Nov 2010 20:54:42 +0100, Oleg Nesterov wrote:
> To simplify, suppose that we have a single-thread tracee, and
> debugger "acks" SIGSTOP, say, it does ptrace(PTRACE_CONT, SIGSTOP).

I do not find this case useful.  It happens with current GDB:
(gdb) handle SIGSTOP
Signal        Stop	Print	Pass to program	Description
SIGSTOP       Yes	Yes	Yes		Stopped (signal)
                                ^^^
But it behaves weird anyway:

ptrace(PTRACE_CONT, 11799, 0x1, SIGSTOP) = 0
wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGSTOP}], WNOHANG, NULL) = 11799
[...]
ptrace(PTRACE_CONT, 11799, 0x1, SIGSTOP) = 0
<no new signal received>:
State:	S (sleeping)
TracerPid:	11797

So the first time it immediately gets reported and the second time it gets
lost.  (kernel-2.6.35.6-48.fc14.x86_64)


> Before this patch, the tracee stops in TASK_STOPPED, now it calls
> ptrace_stop() and goes to TASK_TRACED state.
> 
> Add Jan. I hope this is OK, but this might break the tracer if
> it looks into fs/proc (probably only test-cases do this).

ptrace(PTRACE_CONT, SIGSTOP) should just work somehow so that current GDB does
not break as it calls it sometimes.  The behavior of it may change I guess.

I find only interesting that (PTRACE_DETACH, SIGSTOP) should really keep it
stopped and also the attaching to `T (stopped)' should work with all the
issues of waited-for/unwaited-for SIGSTOP.  Those cases are not being
discussed here.


Thanks,
Jan
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