Re: [PATCH 1/2] file-posix: Add 'accept-read-only-device' option

[Eric Blake <eblake@redhat.com>]

On Wed, Oct 28, 2026 at 06:10:56PM +0100, Kevin Wolf wrote:
> The host_device block driver refuses to open a block device read-write
> if the BLKROGET ioctl tells it that it's actually read-only. For such
> devices it's possible to open a read-write file descriptor, but any
> writes to the device would fail.
> 
> There are valid use cases for which this check gets in the way, as
> explained in the bug report:
> 
>     However, this behavior prevents a legitimate enterprise use case:
>     running VMs against storage replication target LUNs in a disaster
>     recovery configuration. Enterprise storage arrays (Dell Powermax,
>     Pure Storage) set the SCSI Write Protect bit on replication target
>     LUNs, causing the Linux kernel to mark them read-only via BLKROGET.
>     The VMs on the passive DR site need to start with these LUNs
>     attached - writes are expected to fail (the application middleware
>     knows it's on the passive side), and after a DR role swap the LUN
>     becomes writable and writes should succeed immediately without any
>     VM restart.
> 
> In order to allow such use cases in which a currently read-only device
> is deliberately opened read-write, add a new option to the host_device
> driver that allows the user to skip the check.
> 
> Buglink: https://redhat.atlassian.net/browse/RHEL-186136
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  qapi/block-core.json | 19 ++++++++++++++++++-
>  block/file-posix.c   | 23 ++++++++++++++++++++---
>  2 files changed, 38 insertions(+), 4 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org