If the read-only flag is true on a SCSI disk, re-reading the partition
table sets the flag back to false.

To observe this bug, you can run:

1. blockdev --setro /dev/sda
2. blockdev --rereadpt /dev/sda
3. blockdev --getro /dev/sda

This commit reads the disk's old state and combines it with the device
disk-reported state rather than unconditionally marking it as RW.

Reported-by: Li Ning <lining916740...@icloud.com>
Signed-off-by: Jeremy Cline <jer...@jcline.org>
Sorry about this, but there's a bug in the first version of this patch.
I'm not sure what the protocol is for sending revised patches when the
earlier version got accepted, but I don't see the first version in
4.16/scsi-fixes yet. If I should make a patch on top of the v1 patch,
just let me know.

Changes in v2:
  - Don't save the value of "get_disk_ro" to "sdkp->write_prot" since,
    as Li Ning noticed, that causes a second bug: when you make the disk
    writable again with "blockdev --setrw /dev/sda" it isn't possible to
    write to the disk until the partition is re-read.

 drivers/scsi/sd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index bff21e636ddd..3541caf3fceb 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -2595,6 +2595,7 @@ sd_read_write_protect_flag(struct scsi_disk *sdkp, 
unsigned char *buffer)
        int res;
        struct scsi_device *sdp = sdkp->device;
        struct scsi_mode_data data;
+       int disk_ro = get_disk_ro(sdkp->disk);
        int old_wp = sdkp->write_prot;
        set_disk_ro(sdkp->disk, 0);
@@ -2635,7 +2636,7 @@ sd_read_write_protect_flag(struct scsi_disk *sdkp, 
unsigned char *buffer)
                          "Test WP failed, assume Write Enabled\n");
        } else {
                sdkp->write_prot = ((data.device_specific & 0x80) != 0);
-               set_disk_ro(sdkp->disk, sdkp->write_prot);
+               set_disk_ro(sdkp->disk, sdkp->write_prot || disk_ro);
                if (sdkp->first_scan || old_wp != sdkp->write_prot) {
                        sd_printk(KERN_NOTICE, sdkp, "Write Protect is %s\n",
                                  sdkp->write_prot ? "on" : "off");

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