linux-kernel - Re: siginfo pid not populated from ptrace?

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Message-ID: <20181210145716.GC4177@redhat.com>
Date:   Mon, 10 Dec 2018 15:57:16 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Tycho Andersen <tycho@...ho.ws>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: siginfo pid not populated from ptrace?

On 12/06, Tycho Andersen wrote:
>
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1056,11 +1056,14 @@ static int __send_signal(int sig, struct kernel_siginfo *info, struct task_struc
>  		goto ret;
>  
>  	result = TRACE_SIGNAL_DELIVERED;
> +
>  	/*
> -	 * Skip useless siginfo allocation for SIGKILL SIGSTOP,
> -	 * and kernel threads.
> +	 * Skip useless siginfo allocation for SIGKILL and kernel threads.
> +	 * SIGSTOP is visible to tracers, so only skip allocation when the task
> +	 * is not traced.
>  	 */
> -	if (sig_kernel_only(sig) || (t->flags & PF_KTHREAD))
> +	if ((sig == SIGKILL) || (!task_is_traced(t) && sig == SIGSTOP) ||
			          ^^^^^^^^^^^^^^

task_is_traced() checks task->state, probably you meant t->ptrace != 0.

However, in multithreaded case t->ptrace won't help too, unless the signal
is private you do not know which thread will actually dequeue this signal
and possibly report to debugger.

Oleg.