Re: [PATCH v2 1/1] multipathd: verify udev WWIDs with the device before moving paths

[Benjamin Marzinski <bmarzins@redhat.com>]

On Fri, Nov 06, 2026 at 11:19:25PM +0100, Martin Wilck wrote:
> On Fri, 2026-10-30 at 14:09 -0400, Benjamin Marzinski wrote:
> > On Thu, Oct 29, 2026 at 04:52:41PM -0700, Brian Bunker wrote:
> > > multipathd takes a path's WWID from udev. When a change uevent
> > > reports
> > > a different WWID, uev_update_path() removes the path from its map
> > > and
> > > re-adds it through uev_add_path(), which places it in whatever map
> > > owns
> > > the new WWID. Nothing checks the new value against the device
> > > itself;
> > > recheck_wwid only covers paths coming back up in the checker and
> > > "multipathd add path". If udev reports the wrong WWID, or the
> > > uevent is
> > > stale by the time it is processed, the path goes into the map of a
> > > different LUN and I/O for one LUN is silently sent to another.
> > > 
> > > Use SG_IO to read VPD page 0x83 from the device in two places, for
> > > paths where can_recheck_wwid() is true:
> > > 
> > > - In uev_update_path(), if the device still reports the cached
> > > WWID,
> > >   ignore the udev value and leave the path where it is.
> > > 
> > > - In ev_add_path(), when a path is being re-added because its udev
> > >   data was missing or changed, refuse to add it if the device
> > > doesn't
> > >   confirm its WWID. The path is orphaned, its WWID is cleared and
> > > it
> > >   is marked INIT_MISSING_UDEV, so the checker retriggers udev
> > > (bounded
> > >   by retrigger_tries) and the path is re-added from fresh data.
> > > 
> > > To tell re-adds from ordinary adds, turn the need_do_map argument
> > > of
> > > uev_add_path() and ev_add_path() into enum path_add_mode, with a
> > > new
> > > PATH_ADD_REINIT value that behaves like PATH_ADD_MAP and also
> > > requests
> > > the WWID check. Ordinary adds are unchanged and never read the WWID
> > > from the device.
> > > 
> > > If the SG_IO read fails, behavior is unchanged.
> > > 
> > > Split the SG_IO read out of check_path_wwid_change() into
> > > get_sgio_wwid() so all the checks share it.
> > > 
> > > Signed-off-by: Brian Bunker <brian@purestorage.com>
> > Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>
> 
> You will need convince me that this doesn't do more harm than good. 
> We can't just use udev in one place and other methods elsewhere.

I get your concern, but we already do this when recheck_wwid is set.
Perhaps we could also gate this with that config option.
 
> I am open for a general redesign that uses sysfs for retrieving
> WWIDs, rather than udev. But using SG_IO seems wrong to me in general,
> because it can hang. If anything, we'd need to make this configurable,
> and we'd then need to use SG_IO consistently throughout the code (and
> SG_IO has significant disadvantages, as already mentioned). Also, SG_IO
> works for SCSI only. That may be a minor issue these days, because
> almost all disks handled by dm-multipath are SCSI disks, but at least
> formally we still support other block devices like NVMe and DASD.
> 
> Unfortunately there's already a zoo of options for WWID detection which
> has grown over time.
> 
> > About how likely this is: we hit it because a test tool was opening
> > and closing the sd devices read-write many times a second, and with
> > udev's "watch" rule every close generates a change uevent that runs
> > scsi_id against the device. 
> 
> I wouldn't call this a valid test. It's a deliberate abuse of udev
> functionality. If you do this, you need to disable udev's watch
> functionality, which is merely a convenience for users anyway. It's
> questionable if it's a useful feature on production servers in general.
> 
> Thinking about it, I wonder if we should simply set "nowatch" for
> multipath members by default. AFAIU that would avoid the issue Brian
> encountered, at least in this test scenario.

I can't think of anything wrong with disabling that off the top of my
head. Nothing should be directly opening these devices read-write once
they are part of a multipath device.

-Ben
 
> Regards
> Martin
> 
> -- 
> Dr. Martin Wilck <mwilck@suse.com>
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