linux-kernel - Re: [PATCH v2] firmware: fix sending -ERESTARTSYS due to signal on fallback

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Date:   Fri, 26 May 2017 21:46:40 +0200
From:   "Luis R. Rodriguez" <mcgrof@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     "Fuzzey, Martin" <mfuzzey@...keon.com>,
        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 G oede <hdegoede@...hat.com>,
        Alan Cox <alan@...ux.intel.com>, Ted Ts'o <tytso@....edu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] firmware: fix sending -ERESTARTSYS due to signal on
 fallback

On Fri, May 26, 2017 at 06:09:29AM -0500, Eric W. Biederman wrote:
> "Fuzzey, Martin" <mfuzzey@...keon.com> writes:
> >>>> 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?

What do you mean by a killable wait BTW?

ret = swait_event_interruptible_timeout() is being used right now.

The problem is we have:

        if (ret != 0 && fw_st->status == FW_STATUS_ABORTED)                     
                return -ENOENT;                                                 
        if (!ret)                                                               
                return -ETIMEDOUT;                                              
                                                                                
        return ret < 0 ? ret : 0;  

The (!ret) return -ETIMEDOUT ensures that if there was no time left
then we know we ran out of time.

The ret < 0 ? ret makes sure we send any errors
swait_event_interruptible_timeout() sent.

But the caller of this code has:

        if (fw_state_is_aborted(&buf->fw_st))                                   
                retval = -EAGAIN;                                               
        else if (buf->is_paged_buf && !buf->data)                               
                retval = -ENOMEM; 

And this retval is used. so we mask all errors with -EAGAIN.

So Martin is asking us to let us send -ERESTARTSYS back down to drivers.
These potentially could send back down to probe, and so finit_module()
could get this.

Another use case is a custom syfs knob which triggers a request_firmware(),
in such case this is a simple write(), but Anroid is configured to retry
if -ERESTARTSYS so I gather it will *retry* writing again to this file
if -ERESTARTSYS was sent and therefore triggering another firmware request.

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

Can you clarify what you mean?

  Luis