Date:   Fri, 26 May 2017 06:09:29 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     "Fuzzey\, Martin" <mfuzzey@...keon.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        "Michael Kerrisk \(man-pages\)" <mtk.manpages@...il.com>,
        Linux API <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Daniel Wagner <wagi@...om.org>,
        David Woodhouse <dwmw2@...radead.org>,
        jewalt@...innovations.com, rafal@...ecki.pl,
        Arend Van Spriel <arend.vanspriel@...adcom.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        "Li\, Yi" <yi1.li@...ux.intel.com>, atull@...nsource.altera.com,
        Moritz Fischer <moritz.fischer@...us.com>,
        Petr Mladek <pmladek@...e.com>,
        Johannes Berg <johannes.berg@...el.com>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        Luca Coelho <luciano.coelho@...el.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        David Howells <dhowells@...hat.com>,
        Peter Jones <pjones@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>,
        Alan Cox <alan@...ux.intel.com>, "Ted Ts'o" <tytso@....edu>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] firmware: fix sending -ERESTARTSYS due to signal on fallback

"Fuzzey, Martin" <mfuzzey@...keon.com> writes:

> On 25 May 2017 at 06:13, Andy Lutomirski <luto@...nel.org> wrote:
>>>>
>>>> Can you give a simple example of what's going on and why it matters?
>>>>
>
>
> Here is the use case in which I ran into this problem.
>
> I have a driver which does request_firmware() when a write() is done
> to a sysfs file.
>
> The write() was being done by an android init script (with the init
> interpreter "write" command).
> init, of course, forks lots of processes and some of the children die.
>
> So the scenario was the following:
>
> 1) Android init calls write() on the sysfs file
> 2) The sysfs .store() callback registered by a driver is called
> 3) The driver calls request_firmware()
> 4) request_firmware() sends the firmware load request to userspace and
> calls wait_for_completion_interruptible()
> 5) A child dies and raises SIGCHLD
> 6) wait_for_completion_interruptible() returns -ERESTARTSYS due to the signal
> 7) request_firmware() [before this patch] translated that to -EAGAIN
> 8) The driver (in my case) ignored this [because the firmware was not
> critical - it was for checking if a microcontroler was up to date]
> (but it could have returned it to userspace, same problem)
>
> The point being that, due to a signal (SIGCHLD) which has nothing to
> do with the firmware loading process, the firmware load was not done.
> Also EAGAIN is the same error used if the load request times out so it
> was impossible to distinguish the two cases.
>
> ERESTARTSYS is an internal error and is not returned to userspace.
> Instead it is handled by the linux syscall machinery which, after
> processing the signal either restarts (transpently to userspace) the
> syscall or returns EINTR to userspace (depending if the signal handler
> users SA_RESTART - see man 7 signal)
>
>
> With this patch here is what happens:
>
> 1) Android init calls write() on the sysfs file
> 2) The sysfs .store() callback registered by a driver is called
> 3) The driver calls request_firmware()
> 4) request_firmware() sends the firmware load request to userspace and
> calls wait_for_completion_interruptible()
> 5) A child dies and raises SIGCHLD
> 6) wait_for_completion_interruptible() returns -ERESTARTSYS due to the signal
> 7) request_firmware() [with this patch] returns -ERESTARTSYS
> 8) The driver returns -ERSTARTSYS from its sysfs .store method
> 9) The system call machinery invokes the signal handler
> 10) The signal handler does its stuff
> 11) Because SA_RESTART was set the system call is restarted (calling
> the sysfs .store) and we try it all again from step 2
>
> Note that, on the the userspace side  write() is only called once (the
> restart is transparent to userspace which is oblivious to all this)
> The kernel side write() (which calls .store() is called multiple times
> (so that code does need to know about this)
>
>
>>>> ERESTARTSYS and friends are highly magical, and I'm not convinced that
>>>> allowing _request_firmware_load to return -ERESTARTSYS is actually a
>>>> good idea.  What if there are system calls that can't handle this
>>>> style of restart that start being restarted as a result?
>>>
>
> If the caller is unable to restart (for example if the driver's
> .store() callback had already done lots of stuff that couldn't be
> undone) it is free to translate -ERSTARTSYS to -EINTR before
> returning.
> But request_frimware() can't know about that.
>
>
>>>> Maybe SIGCHLD shouldn't interrupt firmware loading?
>
> I don't think there's a way of doing that without disabling all
> signals (ie using the non interruptible wait variants).
> It used to be that way (which is why I only ran into this after
> updating from an ancient 3.16 kernel to a slightly less ancient 4.4)
> But there are valid reasons for wanting to be able to interrupt
> firmware loading (like being able to kill the userspace helper)

Perhaps simply using a killable wait and not a fully interruptible
wait would be better?

It sounds like the code really is not prepared for an truly
interruptible wait here.

Eric

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