linux-kernel - Re: [patch v4 03/27] signal: Get rid of resched

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Message-ID: <874j6119vg.fsf@email.froward.int.ebiederm.org>
Date: Fri, 27 Sep 2024 08:53:07 -0500
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,  Anna-Maria Behnsen
 <anna-maria@...utronix.de>,  Frederic Weisbecker <frederic@...nel.org>,
  John Stultz <jstultz@...gle.com>,  Peter Zijlstra <peterz@...radead.org>,
  Ingo Molnar <mingo@...nel.org>,  Stephen Boyd <sboyd@...nel.org>,  Oleg
 Nesterov <oleg@...hat.com>
Subject: Re: [patch v4 03/27] signal: Get rid of resched_timer logic

Thomas Gleixner <tglx@...utronix.de> writes:

> From: Thomas Gleixner <tglx@...utronix.de>
>
> There is no reason for handing the *resched pointer argument through
> several functions just to check whether the signal is related to a self
> rearming posix timer.
>
> SI_TIMER is only used by the posix timer code and cannot be queued from
> user space.

Huh???  We have rt_sigqueueinfo.  You just touched the code that
copies the queued signal from userspace.

> The only extra check in collect_signal() to verify whether the
> queued signal is preallocated is not really useful. Some other places
> already check purely the SI_TIMER type.

The check to see if the signal was preallocated prevents shenanigans
with setting si_sys_private.

That is today you can queue a signal with rt_sigqueueinfo and set
si_sys_private and it will make it to userspace.  I don't know how
much we care but that is the case.

Which means that WARN_ON you added in __send_signal_locked can
most definitely be triggered by userspace.

Eric


> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>
> ---
>  kernel/signal.c | 25 +++++++++----------------
>  1 file changed, 9 insertions(+), 16 deletions(-)
> ---
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 7706cd304785..3d2e087283ab 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -526,8 +526,7 @@ bool unhandled_signal(struct task_struct *tsk, int sig)
>  	return !tsk->ptrace;
>  }
>  
> -static void collect_signal(int sig, struct sigpending *list, kernel_siginfo_t *info,
> -			   bool *resched_timer)
> +static void collect_signal(int sig, struct sigpending *list, kernel_siginfo_t *info)
>  {
>  	struct sigqueue *q, *first = NULL;
>  
> @@ -549,12 +548,6 @@ static void collect_signal(int sig, struct sigpending *list, kernel_siginfo_t *i
>  still_pending:
>  		list_del_init(&first->list);
>  		copy_siginfo(info, &first->info);
> -
> -		*resched_timer =
> -			(first->flags & SIGQUEUE_PREALLOC) &&
> -			(info->si_code == SI_TIMER) &&
> -			(info->si_sys_private);
> -
>  		__sigqueue_free(first);
>  	} else {
>  		/*
> @@ -571,13 +564,12 @@ static void collect_signal(int sig, struct sigpending *list, kernel_siginfo_t *i
>  	}
>  }
>  
> -static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
> -			kernel_siginfo_t *info, bool *resched_timer)
> +static int __dequeue_signal(struct sigpending *pending, sigset_t *mask, kernel_siginfo_t *info)
>  {
>  	int sig = next_signal(pending, mask);
>  
>  	if (sig)
> -		collect_signal(sig, pending, info, resched_timer);
> +		collect_signal(sig, pending, info);
>  	return sig;
>  }
>  
> @@ -589,17 +581,15 @@ static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
>  int dequeue_signal(sigset_t *mask, kernel_siginfo_t *info, enum pid_type *type)
>  {
>  	struct task_struct *tsk = current;
> -	bool resched_timer = false;
>  	int signr;
>  
>  	lockdep_assert_held(&tsk->sighand->siglock);
>  
>  	*type = PIDTYPE_PID;
> -	signr = __dequeue_signal(&tsk->pending, mask, info, &resched_timer);
> +	signr = __dequeue_signal(&tsk->pending, mask, info);
>  	if (!signr) {
>  		*type = PIDTYPE_TGID;
> -		signr = __dequeue_signal(&tsk->signal->shared_pending,
> -					 mask, info, &resched_timer);
> +		signr = __dequeue_signal(&tsk->signal->shared_pending, mask, info);
>  
>  		if (unlikely(signr == SIGALRM))
>  			posixtimer_rearm_itimer(tsk);
> @@ -626,7 +616,7 @@ int dequeue_signal(sigset_t *mask, kernel_siginfo_t *info, enum pid_type *type)
>  	}
>  
>  	if (IS_ENABLED(CONFIG_POSIX_TIMERS)) {
> -		if (unlikely(resched_timer))
> +		if (unlikely(info->si_code == SI_TIMER && info->si_sys_private))
>  			posixtimer_rearm(info);
>  	}
>  
> @@ -1011,6 +1001,9 @@ static int __send_signal_locked(int sig, struct kernel_siginfo *info,
>  
>  	lockdep_assert_held(&t->sighand->siglock);
>  
> +	if (WARN_ON_ONCE(!is_si_special(info) && info->si_code == SI_TIMER))
> +		return 0;
> +
>  	result = TRACE_SIGNAL_IGNORED;
>  	if (!prepare_signal(sig, t, force))
>  		goto ret;