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 20:42:44 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Oleg Nesterov <oleg@...hat.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 Mon, Feb 14, 2011 at 8:01 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> 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.

Yes, I understand what happens.

Basically, we have TASK_RUNNING, TASK_STOPPED and TASK_TRACED
states, and after entering TASK_TRACED state we lose information
in which state we were before entering it. We need to remember
old state and restore it in order for this example to work.

Or we can avoid entering TASK_TRACED on ptrace(PTRACE_GETSIGINFO) et al.
Can we remain in TASK_STOPPED?

-- 
vda
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