linux-kernel - Re: [PATCH v2] signal: Adjust error codes according to restore_user

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Date:   Wed, 29 May 2019 18:57:18 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Deepa Dinamani <deepa.kernel@...il.com>
Cc:     David Laight <David.Laight@...lab.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        "dbueso@...e.de" <dbueso@...e.de>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        Davidlohr Bueso <dave@...olabs.net>, Eric Wong <e@...24.org>,
        Jason Baron <jbaron@...mai.com>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        linux-aio <linux-aio@...ck.org>,
        Omar Kilani <omar.kilani@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH v2] signal: Adjust error codes according to
 restore_user_sigmask()

On 05/28, Deepa Dinamani wrote:
>
> I agree that signal handller being called and return value not being
> altered is an issue with other syscalls also. I was just wondering if
> some userspace code assumption would be assuming this. This is not a
> kernel bug.
>
> But, I do not think we have an understanding of what was wrong in
> 854a6ed56839a anymore since you pointed out that my assumption was not
> correct that the signal handler being called without errno being set
> is wrong.

Deepa, sorry, I simply can't parse the above... most probably because of
my bad English.

> One open question: this part of epoll_pwait was already broken before
> 854a6ed56839a. Do you agree?
>
> if (err == -EINTR) {
>                    memcpy(&current->saved_sigmask, &sigsaved,
>                           sizeof(sigsaved));
>                     set_restore_sigmask();
>   } else
>                    set_current_blocked(&sigsaved);

I do not understand why do you think this part was broken :/

> Or, I could revert the signal_pending() check and provide a fix
> something like below(not a complete patch)

...

> -void restore_user_sigmask(const void __user *usigmask, sigset_t *sigsaved)
> +int restore_user_sigmask(const void __user *usigmask, sigset_t
> *sigsaved, int sig_pending)
>  {
> 
>         if (!usigmask)
>                return;
> 
>         /*
>          * 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 (sig_pending) {
>                 current->saved_sigmask = *sigsaved;
>                 set_restore_sigmask();
>                return;
>            }
> 
> @@ -2330,7 +2330,8 @@ SYSCALL_DEFINE6(epoll_pwait, int, epfd, struct
> epoll_event __user *, events,
> 
>         error = do_epoll_wait(epfd, events, maxevents, timeout);
> 
> -       restore_user_sigmask(sigmask, &sigsaved);
> +       signal_detected = restore_user_sigmask(sigmask, &sigsaved,
> error == -EINTR);

I fail to understand this pseudo-code, sorry. In particular, do not understand
why restore_user_sigmask() needs to return a boolean.

The only thing I _seem to_ understand is the "sig_pending" flag passed by the
caller which replaces the signal_pending() check. Yes, this is what I think we
should do, and this is what I tried to propose from the very beginning in my
1st email in this thread.

Oleg.