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

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Date:	Mon, 14 Feb 2011 20:01:41 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Tejun Heo <tj@...nel.org>, Roland McGrath <roland@...hat.com>,
	jan.kratochvil@...hat.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org
Subject: Re: [PATCH 1/1] ptrace: make sure do_wait() won't hang after
	PTRACE_ATTACH

On 02/14, Denys Vlasenko wrote:
>
> I tried to simply not do ptrace(PTRACE_SYSCALL, ..., 0) at all.
> Behavior changes, but it is still wrong. Now tracee doesn't wake up
> on SIGCONT.

please see below,

> 19:41:09.601764 wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGSTOP}],
> __WALL, NULL) = 25339
> 19:41:09.601914 rt_sigprocmask(SIG_BLOCK, [HUP INT QUIT PIPE TERM], NULL, 8) = 0
> 19:41:09.602081 ptrace(PTRACE_GETSIGINFO, 25339, 0, {si_signo=SIGSTOP,
> si_code=SI_USER, si_pid=10105, si_uid=0, si_value={int=0, ptr=0}}) = 0
> 19:41:09.602273 write(2, "--- SIGSTOP (Stopped (signal)) @ 0 (0) ---
> STOP: si_signo:19 si_code:0 si_status:0 si_value:(nil) \n", 99) = 99
> 19:41:09.602456 ptrace(PTRACE_SYSCALL, 25339, 0x1, SIGSTOP) = 0
> 19:41:09.602582 --- SIGCHLD (Child exited) @ 0 (0) ---
> 19:41:09.602652 rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
> 19:41:09.602792 wait4(-1, [{WIFSTOPPED(s) && WSTOPSIG(s) == SIGSTOP}],
> __WALL, NULL) = 25339

OK, it is stopped.

> 19:41:09.603081 ptrace(PTRACE_GETSIGINFO, 25339, 0, 0x7fff436fc730) =

And this changes the state to TASK_TRACED. See another email from me.
That is why SIGCONT doesn't work.

This is another problem, the kernel should help somehow. This was
already discussed a bit, but it is not clear what exactly we can do.

Oleg.

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