linux-kernel - Re: [PATCH] signal: remove the wrong signal_pending() check in restore_user

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Message-ID: <20190605092516.GC32406@redhat.com>
Date:   Wed, 5 Jun 2019 11:25:17 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Davidlohr Bueso <dbueso@...e.de>, Jens Axboe <axboe@...nel.dk>,
        Davidlohr Bueso <dave@...olabs.net>,
        "e@...24.org" <e@...24.org>, Jason Baron <jbaron@...mai.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-aio@...ck.org" <linux-aio@...ck.org>,
        "omar.kilani@...il.com" <omar.kilani@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        stable <stable@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] signal: remove the wrong signal_pending() check in
 restore_user_sigmask()

On 06/05, David Laight wrote:
>
> epoll() would have:
> 	if (restore_user_sigmask(xxx.sigmask, &sigsaved, !ret || ret == -EINTR))
> 		ret = -EINTR;

I don't think so but lets discuss this later.

> I also think it could be simplified if code that loaded the 'user sigmask'
> saved the old one in 'current->saved_sigmask' (and saved that it had done it).
> You'd not need 'sigsaved' nor pass the user sigmask address into
> the restore function.

Heh. apparently you do not read my emails ;)

This is what I proposed in my very 1st email, and I even showed the patch
and the code with the patch applied twice. Let me do this again.

Let me show the code with the patch applied. I am using epoll_pwait() as an
example because it looks very simple.


	static inline void set_restore_sigmask(void)
	{
// WARN_ON(!TIF_SIGPENDING) was removed by this patch
		current->restore_sigmask = true;
	}

	int set_xxx(const sigset_t __user *umask, size_t sigsetsize)
	{
		sigset_t *kmask;

		if (!umask)
			return 0;
		if (sigsetsize != sizeof(sigset_t))
			return -EINVAL;
		if (copy_from_user(kmask, umask, sizeof(sigset_t)))
			return -EFAULT;

// we can safely modify ->saved_sigmask/restore_sigmask, they has no meaning
// until the syscall returns.
		set_restore_sigmask();
		current->saved_sigmask = current->blocked;
		set_current_blocked(kmask);

		return 0;
	}


	void update_xxx(bool interrupted)
	{
// the main reason for this helper is WARN_ON(!TIF_SIGPENDING) which was "moved"
// from set_restore_sigmask() above.
		if (interrupted)
			WARN_ON(!test_thread_flag(TIF_SIGPENDING));
		else
			restore_saved_sigmask();
	}

	SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct epoll_event __user *, events,
			int, maxevents, int, timeout, const sigset_t __user *, sigmask,
			size_t, sigsetsize)
	{
		int error;

		error = set_xxx(sigmask, sigsetsize);
		if (error)
			return error;

		error = do_epoll_wait(epfd, events, maxevents, timeout);
		update_xxx(error == -EINTR);

		return error;
	}

Oleg.