"mdev: remove undocumented subsystem/devname matching hack" disaster.

[Piotr Karbowski]

On 04/02/2013 03:46 PM, Denys Vlasenko wrote:
> On Tue, Apr 2, 2013 at 3:31 PM, Piotr Karbowski
> <piotr.karbowski at gmail.com>wrote:
>
>> On 04/02/2013 02:35 PM, Denys Vlasenko wrote:
>>
>>> The invocation which sets wrong ownership has $DEVNAME set:
>>>
>>> mdev[730]: 56.531462 ACTION:add SUBSYSTEM:sound DEVNAME:snd/controlC0
>>> DEVPATH:/devices/pci0000:00/**0000:00:1b.0/sound/card0/**controlC0
>>> mdev[730]: dev 116,5
>>> ...
>>>
>>> If $DEVNAME is set, we don't guess device name to be basename
>>> of $DEVPATH, we use $DEVNAME instead
>>> (because it looks like a better choice).
>>>
>>> "snd/controlC0" won't match "control.*       root:audio 660 =snd/"
>>> Last catch-all rule gets matched instead:
>>>
>>> mdev[730]: rule matched, line 111
>>> mdev[730]: mknod snd/controlC0 (116,5) 20660 0:0
>>> mdev[730]: running: /opt/mdev/helpers/catch-all
>>>
>>> This looks less than ideal, but I don't see immediate ways
>>> to improve it. Ideas?
>>>
>>>
>> Well, that's awkward, because mdev -s  see 'controlC0' but the mdev as
>> hotplug agent after modprobe see it as snd/controlC0. I can imagine that
>> this affect everything not only sound system. Would it be possible to
>> restore how it was handled in older mdev? Otherwise that would require some
>> heavy lifting regexes to handle it and some of the results can be far from
>> desired result.
>>
>
> It would require duplication of rules, yes.
> I don't like this detail either.
>
> In some cases you can have one rule
> by matching $DEVPATH:
> DEVPATH=.*/control.*;.*   ....

reworking most if not all rules with DEVPATH= does not looks really nice.

>
>> Woudn't it be better to 'basename' it? Because if the module is loaded or
>> compiled in-kernel, it will be just 'controlC0', and I was pretty sure
>> kernel never-ever provide subdir in DEVNAME,
>>
>
> Looks like kernel provides subdirs in $DEVNAME (now).
>
>
>> even the msr/cpuid support in kernel with help desc about /dev/cpu used to
>> be created in bare /dev. The old mdev was using basename, and I see no real
>> reason to not keep this as-it-was, what you think?
>>
>
>>From $DEVNAMEs I saw it looks like using it makes sense:
> kernel already tells us the subdir, no need to code it up
> in the rules.

If so, then mdev -s needs a way to support this, somehow to get the 
snd/X, if that's not possible, I would highly suggest to basename $MDEV.

Other, ugly but possible workaround would be for me to put a thingy in 
catch-all script to check if $MDEV contain slash (and since it hit 
catch-all it wasn't handled), if it does, then basename it, export MDEV, 
DEVPATH etc and exec /sbin/mdev. Sounds better than in-mdev basenaming it?

-- Piotr.