Re: Moving from qemu-pr-helper and libmpathpersist to <linux/pr.h>

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Feb 09, 2026 at 01:50:00PM +0100, Hannes Reinecke wrote:
> On 2/6/26 15:08, Stefan Hajnoczi wrote:
> > On Fri, Feb 06, 2026 at 01:03:18AM +0100, Hannes Reinecke wrote:
> > > On 2/5/26 14:39, Stefan Hajnoczi wrote:
> [ .. ]
> > > That would make sense, but unfortunately READ KEYS (and READ
> > > RESERVATIONS) requires the same privileges than the other
> > > blkpr functions. Might be an idea to change that, though.
> > 
> > Making sure I understand:
> > 
> > blkdev_pr_read_keys() and blkdev_pr_read_reservation() in Linux
> > block/ioctl.c should be adjusted to allow not just BLK_OPEN_WRITE but
> > also BLK_OPEN_READ?
> > 
> Yes.
> 
> > I think it's okay from a security perspective: if the application can
> > already read the entire disk, then it's okay for it to read the keys and
> > reservation information. But I'm not 100% sure...
> > 
> > In any case, I think this idea is orthogonal to the discussion about
> > DM-Multipath <linux/pr.h> support. In terms of permission requirements,
> > <linux/pr.h> already requires fewer permissions (just opening the block
> > device for write) today than libmpathpersist or ioctl(SG_IO). Or do you
> > see a scenario where the application wants to open the block device as
> > root (or CAP_SYS_RAWIO) but read-only, so requiring opening the device
> > with write permissions is a blocker?
> > 
> 
> My concern with opening the block device with BLK_OPEN_WRITE is that
> this will trigger udev to 'synthesize' (ie regenerate) an 'add' event
> on close, causing 'interesting' effects as this will cascade down
> through the udev rule chain, triggering blkid, partition scan, you
> name it.
> Horrible, horrible, horrible.
> Don't do it.
> 
> Especially not as you are only interested in reading information,
> and not changing the disk state in any way.

I see. I will send patches to change blkdev_pr_read_keys()
blkdev_pr_read_reservation() to require only BLK_OPEN_READ.

Stefan

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