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From:   Deepa Dinamani <deepa.kernel@gmail.com>
Date:   Thu, 23 May 2019 13:41:17 -0700
Message-ID: <CABeXuvo5Y0aHgo-xMzmW7V02g+ysGqAkdoCAkW7L6LkukdvAcg@mail.gmail.com>
Subject: Re: [PATCH v2] signal: Adjust error codes according to restore_user_sigmask()
To:     David Laight <David.Laight@aculab.com>
Cc:     Oleg Nesterov <oleg@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
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        "dbueso@suse.de" <dbueso@suse.de>,
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        Davidlohr Bueso <dave@stgolabs.net>, Eric Wong <e@80x24.org>,
        Jason Baron <jbaron@akamai.com>,
        Linux FS-devel Mailing List <linux-fsdevel@vger.kernel.org>,
        linux-aio <linux-aio@kvack.org>,
        Omar Kilani <omar.kilani@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
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On Thu, May 23, 2019 at 11:06 AM Deepa Dinamani <deepa.kernel@gmail.com> wrote:
>
> Ok, since there has been quite a bit of argument here, I will
> backtrack a little bit and maybe it will help us understand what's
> happening here.
> There are many scenarios being discussed on this thread:
> a. State of code before 854a6ed56839a
> b. State after 854a6ed56839a
> c. Proposed fix as per the patchset in question.
>
> Oleg, I will discuss these first and then we can discuss the
> additional changes you suggested.
>
> Some background on why we have these syscalls that take sigmask as an
> argument. This is just for the sake of completeness of the argument.
>
> These are particularly meant for a scenario(d) such as below:
>
> 1. block the signals you don't care about.
> 2. syscall()
> 3. unblock the signals blocked in 1.
>
> The problem here is that if there is a signal that is not blocked by 1
> and such a signal is delivered between 1 and 2(since they are not
> atomic), the syscall in 2 might block forever as it never found out
> about the signal.
>
> As per [a] and let's consider the case of epoll_pwait only first for simplicity.
>
> As I said before, ep_poll() is what checks for signal_pending() and is
> responsible for setting errno to -EINTR when there is a signal.
>
> So if a signal is received after ep_poll() and ep_poll() returns
> success, it is never noticed by the syscall during execution.
> So the question is does the userspace have to know about this signal
> or not. From scenario [d] above, I would say it should, even if all
> the fd's completed successfully.
> This does not happen in [a]. So this is what I said was already broken.
>
> What [b] does is to move the signal check closer to the restoration of
> the signal. This way it is good. So, if there is a signal after
> ep_poll() returns success, it is noticed and the signal is delivered
> when the syscall exits. But, the syscall error status itself is 0.
>
> So now [c] is adjusting the return values based on whether extra
> signals were detected after ep_poll(). This part was needed even for
> [a].
>
> Let me know if this clarifies things a bit.

Just adding a little more clarification, there is an additional change
between [a] and [b].
As per [a] we would just restore the signal instead of changing the
saved_sigmask and the signal could get delivered right then. [b]
changes this to happen at syscall exit:

void restore_user_sigmask(const void __user *usigmask, sigset_t *sigsaved)
{

           <snip>

          /*
           * When signals are pending, do not restore them here.
           * Restoring sigmask here can lead to delivering signals
that the above
           * syscalls are intended to block because of the sigmask passed in.
           */
           if (signal_pending(current)) {
           current->saved_sigmask = *sigsaved;
           set_restore_sigmask();
           return;
}

-Deepa