[dm-devel] [RFC PATCH 0/2] multipath: change default devnode blacklist

[Benjamin Marzinski]

On Fri, Jun 05, 2020 at 07:20:26PM +0000, Martin Wilck wrote:
> Hi Ben,
> 
> On Thu, 2020-06-04 at 19:30 -0500, Benjamin Marzinski wrote:
> > I recently got a request to add the Oracle ASM filer driver devices
> > to
> > multipath's builtin devnode blacklist.  However, instead of having to
> > do
> > always this for each device type individually, I decided to make
> > multipath blacklist all non scsi, nvme, and dasd devnodes by default.
> > This is what the multipath udev rules already do. If people don't
> > like
> > this solution, the alternative is to add another line to the default
> > devnode blacklist like "^(asm/|oracleafd/|ofsctl)".
> 
> Thanks, this looks ok. But I'd like to propose an alternative idea:
> Extend the RE syntax in our config file to allow negated regular 
> expressions. Like this:
> 
> blacklist {
>     devnode "!(^(sd[a-z]|dasd[a-z]|nvme[0-9]))"
> }
> 
> The "!(${RE})" construct would mean "everything that does not match ${RE}".
> This logic would only be applied to an entire regex.
> If a user needs a RE matching with "!(" and ending with ")", she can escape 
> the exclamation mark "\!(like this)".
> 
> AFAICS this could be implemented quite easily (by adding a "bool negate" field
> in struct blentry and some simple parser logic), and could be applied to other 
> REs in the config file as well. We could print this with "multipath -t", and 
> we wouldn't need to document an exception.
> 
> It's also pretty much backwards-compatible, I don't think many people use
> regexes starting with "!(" for multipath these days.

Sure, but since we can only really support negating the whole regular
expression, and c regular expressions don't treat '!' as a special
character, why do we need the prentheses around the regular expression?
It seems like we can just treat regular expressions starting with '!' as
negated, and ones starting with "\!" as starting with a literal '!'.

Do you think that there is much chance that users have blacklist strings
that start with '!'? There are no devnodes, udev properties, or
protocols that start with that.  I don't know of a UUID format that has
an exclamation point, and while it's possible that a product string
starts with one, it seems really unlikely.

-Ben

> 
> Regards,
> Martin
> 
> -- 
> Dr. Martin Wilck <mwilck at suse.com>, Tel. +49 (0)911 74053 2107
> SUSE  Software Solutions Germany GmbH
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