Re: IBM request to allow unprivledged ioctls [Was: Revert "dm mpath: fix stalls when handling invalid ioctls"]

[Paolo Bonzini <pbonzini@redhat.com>]

On 29/10/2015 14:18, Mike Snitzer wrote:
> > 4) dmesg shows that scsi_verify_blk_ioctl() failed for SG_IO (0x2285);
> >    it returns -ENOIOCTLCMD, later replaced with -ENOTTY in vfs_ioctl().
> > 
> >     $ dmesg
> >     <...>
> >     [] device-mapper: multipath: Failing path 65:144.
> >     [] device-mapper: multipath: Failing path 67:144.
> >     [] device-mapper: multipath: Failing path 65:224.
> >     [] device-mapper: multipath: Failing path 68:32.
> >     [] sgio_inquiry: sending ioctl 2285 to a partition!
> 
> So scsi_verify_blk_ioctl() considers the ioctl invalid.

But that's wrong, I think.  It's a false positive in
scsi_verify_blk_ioctl().

If the ioctl is valid when bdev becomes non-NULL (and it will be if
ti->len becomes equal to i_size_read(bdev->bd_inode) >> SECTOR_SHIFT),
you should not return -ENOIOCTLCMD aka ENOTTY, because userspace doesn't
think the ioctls can go away and come back.  So Hannes's patch broke the
userspace ABI. :(

Paolo