pr-helper: fix assertion failure on failed multipath PERSISTENT RESER…

…VE IN

The response size is expected to be zero if the SCSI status is not
"GOOD", but nothing was resetting it.

This can be reproduced simply by "sg_persist -s /dev/sdb" where /dev/sdb
in the guest is a scsi-block device corresponding to a multipath device
on the host.

Before:

  PR in (Read full status): Aborted command

and on the host:

  prh_write_response: Assertion `resp->sz == 0' failed.

After:

  PR in (Read full status): bad field in cdb or parameter list
  (perhaps unsupported service action)

Reported-by: Jiri Belka <[email protected]>
Reviewed-by: Michal Privoznik <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>

scsi/qemu-pr-helper.c

@@ -558,7 +558,11 @@ static int do_pr_in(int fd, const uint8_t *cdb, uint8_t *sense,
 #ifdef CONFIG_MPATH
     if (is_mpath(fd)) {
         /* multipath_pr_in fills the whole input buffer.  */
-        return multipath_pr_in(fd, cdb, sense, data, *resp_sz);
+        int r = multipath_pr_in(fd, cdb, sense, data, *resp_sz);
+        if (r != GOOD) {
+            *resp_sz = 0;
+        }
+        return r;
     }
 #endif