[PATCH] scsi: sr: propagate read-only status to block layer via set_disk_ro()

[Daan De Meyer <daan.j.demeyer@gmail.com>]

The sr driver tracks whether a CD-ROM drive supports writing via the
cd->writeable flag, which is determined from the drive's MODE SENSE
capabilities page (page 0x2A). When the drive does not report any write
capabilities (CD-RW, DVD-RAM, MRW-W, etc.), writeable remains 0 and
write requests are rejected in sr_init_command().

However, this information is never propagated to the block layer via
set_disk_ro(). As a result, BLKROGET on a CD-ROM device always returns
0 (writable), even when the drive has no write capabilities and writes
will inevitably fail. This causes problems for userspace that relies on
BLKROGET to determine whether a block device is read-only, for example:

 - systemd's loop device setup uses BLKROGET to decide whether to create
   a loop device with LO_FLAGS_READ_ONLY. Without the read-only flag,
   writes pass through the loop device to the CD-ROM and fail with I/O
   errors.

 - systemd-fsck checks BLKROGET to decide whether to run fsck in
   no-repair mode (-n). Without the read-only flag, fsck attempts writes
   that produce I/O errors.

This is in contrast to the sd driver (SCSI disks), which has called
set_disk_ro() based on the MODE SENSE Write Protect bit since before
the beginning of the git history.

Fix this by calling set_disk_ro(disk, !cd->writeable) in sr_probe()
after get_capabilities() has determined the drive's write support.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Daan De Meyer <daan.j.demeyer@gmail.com>
---
 drivers/scsi/sr.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/scsi/sr.c b/drivers/scsi/sr.c
index 7adb2573f50d..8ed002f82e36 100644
--- a/drivers/scsi/sr.c
+++ b/drivers/scsi/sr.c
@@ -684,6 +684,7 @@ static int sr_probe(struct scsi_device *sdev)
 	sr_vendor_init(cd);
 
 	set_capacity(disk, cd->capacity);
+	set_disk_ro(disk, !cd->writeable);
 	disk->private_data = cd;
 
 	if (register_cdrom(disk, &cd->cdi))
-- 
2.53.0