Message-ID: <CAEJqkgjvOBagdSNhg04466vUXq7v55fE4vHvF66JjezDLu+wFg@mail.gmail.com>
Date:   Mon, 24 Dec 2018 12:32:08 +0100
From:   Gabriel C <nix.or.die@...il.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Christian Brauner <christian@...uner.io>,
        Marcus Meissner <christian.brauner@...onical.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: FYI: Userland breakage caused by udev bind commit

Am Mo., 24. Dez. 2018 um 11:54 Uhr schrieb Greg KH <gregkh@...uxfoundation.org>:
>
> On Mon, Dec 24, 2018 at 11:15:34AM +0100, Gabriel C wrote:
> > Am Mo., 24. Dez. 2018 um 10:17 Uhr schrieb Greg KH <gregkh@...uxfoundation.org>:
> > >
> > > On Mon, Dec 24, 2018 at 08:31:27AM +0100, Gabriel C wrote:
> > > > Am So., 23. Dez. 2018 um 19:09 Uhr schrieb Dmitry Torokhov
> > > > <dmitry.torokhov@...il.com>:
> > > >
> > > > [ also added Linus to CC on that one too ]
> > > > >
> > > > > On Sun, Dec 23, 2018 at 06:17:04PM +0100, Christian Brauner wrote:
> > > > > > On Sun, Dec 23, 2018 at 05:49:54PM +0100, Marcus Meissner wrote:
> > > > > > > Hi,
> > > > > > >
> > > > > > > I am the maintainer of libmtp and libgphoto2
> > > > > > >
> > > > > > > Some months ago I was made aware of this bug:
> > > > > > >     https://bugs.kde.org/show_bug.cgi?id=387454
> > > > > > >
> > > > > > > This was fallout identified to come from this kernel commit:
> > > > > > >
> > > > > > >     commit 1455cf8dbfd06aa7651dcfccbadb7a093944ca65
> > > > > > >     Author: Dmitry Torokhov <dmitry.torokhov@...il.com>
> > > > > > >     Date:   Wed Jul 19 17:24:30 2017 -0700
> > > > > >
> > > > > > Fwiw, the addition of {un}bind events has caused issues for
> > > > > > systemd-udevd as well and is tracked here:
> > > > > > https://github.com/systemd/systemd/issues/7587
> > > > > > I haven't been aware of this until yesterday and it seems that so far
> > > > > > this hasn't been brought up on lkml until you did now.
> > > > >
> > > > > The fallout was caused by premature enabling of the new events in
> > > > > systemd/udev by yours truly (even though the commit has Lennart's name
> > > > > on it due to how it was merged):
> > > > >
> > > > > https://github.com/systemd/systemd/commit/9a39e1ce314d1a6f8a754f6dab040019239666a9
> > > > >
> > > > > "Add handling for bind/unbind actions (#6720)
> > > > >
> > > > > Newer kernels will emit uevents with "bind" and "unbind" actions. These
> > > > > uevents will be issued when driver is bound to or unbound from a device.
> > > > > "Bind" events are helpful when device requires a firmware to operate
> > > > > properly, and driver is unable to create a child device before firmware
> > > > > is properly loaded.
> > > > >
> > > > > For some reason systemd validates actions and drops the ones it does not
> > > > > know, instead of passing them on through as old udev did, so we need to
> > > > > explicitly teach it about them."
> > > > >
> > > > > Similarly it is now papered over in systemd/udev until we make it
> > > > > properly handle new events:
> > > > >
> > > > > https://github.com/systemd/systemd/commit/56c886dc7ed5b2bb0882ba85136f4070545bfc1b
> > > > >
> > > > > "sd-device: ignore bind/unbind events for now
> > > > >
> > > > > Until systemd/udev are ready for the new events and do not flush entire
> > > > > device state on each new event received, we should ignore them."
> > > > >
> > > >
> > > > And how about peoples still uses systemd < 235 and newer kernels ?
> > >
> > > Is that an issue?  Who uses that, and does it cause problems on their
> > > systems given that the events just do not do anything for those systems?
> > >
> > > We tested this out a lot back in the summer of 2017 and I thought all
> > > was well.  What recently changed that caused breakages to suddenly show
> > > up?  How have we not seen this until now?
> > >
> >
> > Well people observed that , please click the bug link for that KDE bug.
> > Reported '2017-11-30'..
> >
> > I can reproduce that on systemd 231 ( which we have here ) and
> > kernels >= 4.14 just easy.
> >
> > Can't use any mtp devices all dropping :
> >
> > The file or folder udi=/org/kde/solid/udev/.......  does not exists'
> >
> > Why it got not reported here is probably because people are shy to
> > report such things to LKML.
> >
> > > We can drop the "new" uevents now by reverting the patch, but what about
> > > the userspace tools that now depend on them as we have had them in our
> > > kernels for so long?  We can't now break them, right?  Should we add a
> > > new kernel config option to not emit those for older userspaces that can
> > > not handle this (of which I really still do not understand given that we
> > > tested the heck out of this last year...)
> >
> > Peoples started to add workarounds to make it work somewhat again.
> >
> > Greg any such changes to udev are very fragile.
>
> I am not changing udev.  Well, Dmitry changed udev, and then reverted
> it, so all should be fine :)
>
> > Also dropping some patch to systemd-udev won't solve anything on such moves.
>
> If systemd-udev was broken, it should resolve the issue, right?
>
> > Remember there exists other udev impelmentations too and not only that.
>
> Ok, what other udev implementations are broken and why have we not heard
> from them in the past 1 1/2 years?

That because software maintainers started to change / workaround with
some different set of
udev rules :)

Original report was on Gentoo and eudev also ..

An now with systemd 240 it will break again probably =)

>
> > See example below :
> >
> > app1- xxx - depending on some udev / kernel behaviour ( add rule in this case )
> > kernel - xxx changes that ( adding bind which confuses add to usersapce )
>
> No, another random uevent should never confuse userspace as userspace
> always had to properly handle any uevent it got, no matter what it was
> called.  Why would userspace get confused?

See : https://bugs.kde.org/show_bug.cgi?id=387454#c29

>
> > - on update to that kernel app1 breaks..
> > - udevd - drops an patch in to catch up
> > - app1 trying to workaround now both ( which is that case here )
> >    and now here the mess starts.
>
> What application is working around what exactly?  Specific patches would
> be good to point to.
>

In case of libmtp / gphoto2 , so far I can tell they changed the udev rules.
However Marcus Meissner should know better than me :)

In case of solid .. is still broken ..

> > Having app1-fixed for kernel who changed behaviour and using now
> > and kernel does not have this makes app1 breaks again
> >
> > Using fixed udev and app1 without workarounds on kernel with bind breaks,
> > using not fixed udev , app1 without workround breaks etc..
> >
> > >
> > > still confused,
> > >
> >
> > The problem I see here is 'bind' confuses 'add'.
> >
> > So is there a way to make bind event _not_ confusing add event ?
>
> A bind event should not confuse any other events at all, it is as if
> adding any other type of uevent would also confuse an add event?
>
> Something is really wrong if that were to happen why is udev thinking
> 'bind' is the same as 'add'?  Is it also thinking that 'unbind' is the
> same as 'add'?
>

I don't know why that is and cannot answer you that yet.
I'm near 900KM away from my place and don't have the right hardware
near me to debug/look
why some software gets confused.

> And see Dmitry's email, it seems that all of the combinations are now
> handled properly.
>
> If not, how to resolve this?

I don't know how Dimitry's testing looks like on this however
since I can reproduce that bug with systemd 231 and kernel >= 4.14
I don't think that is resolved.

Also he talks just about systemd's udev .. What about others ?

I don't know for sure on how that can be solved , reverting this patch
seems to be
not a good idea.

Maybe your idea adding a CONFIG about , enable it by default so software
at least can have a sort check on the kernel configuration and based on that
taking action on what rules to install and so on ?

>
> thanks,
>
> greg k-h

BR,

Gabriel

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