From bcbf10c15f43cfc079622c25ef282688965a7128 Mon Sep 17 00:00:00 2001
From: Paolo Bonzini <pbonzini@redhat.com>
Date: Tue, 13 Nov 2012 10:34:17 +0100
Subject: [PATCH] nbd: fixes to read-only handling

We do not need BLKROSET if the kernel supports setting flags.
Also, always do BLKROSET even for a read-write export, otherwise
the read-only state remains "sticky" after the invocation of
"qemu-nbd -r".

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit c8969eded252058e90e91f12f75f32aceae46ec9)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
---
 nbd.c | 25 ++++++++++++-------------
 1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/nbd.c b/nbd.c
index 206f75c..19f6cd8 100644
--- a/nbd.c
+++ b/nbd.c
@@ -399,24 +399,23 @@ int nbd_init(int fd, int csock, uint32_t flags, off_t size, size_t blocksize)
         return -serrno;
     }
 
-    if (flags & NBD_FLAG_READ_ONLY) {
-        int read_only = 1;
-        TRACE("Setting readonly attribute");
-
-        if (ioctl(fd, BLKROSET, (unsigned long) &read_only) < 0) {
+    if (ioctl(fd, NBD_SET_FLAGS, flags) < 0) {
+        if (errno == ENOTTY) {
+            int read_only = (flags & NBD_FLAG_READ_ONLY) != 0;
+            TRACE("Setting readonly attribute");
+
+            if (ioctl(fd, BLKROSET, (unsigned long) &read_only) < 0) {
+                int serrno = errno;
+                LOG("Failed setting read-only attribute");
+                return -serrno;
+            }
+        } else {
             int serrno = errno;
-            LOG("Failed setting read-only attribute");
+            LOG("Failed setting flags");
             return -serrno;
         }
     }
 
-    if (ioctl(fd, NBD_SET_FLAGS, flags) < 0
-        && errno != ENOTTY) {
-        int serrno = errno;
-        LOG("Failed setting flags");
-        return -serrno;
-    }
-
     TRACE("Negotiation ended");
 
     return 0;