Linux block devices can be set read-only with "blockdev --setro
<device>". The same thing can be done for LVM volumes using "lvchange
--permission r <volume>". This read-only setting is independent of
device node permissions. Therefore the device can still be opened
O_RDWR but actual writes will fail.
This results in odd behavior for QEMU. bdrv_open() is supposed to fail
if a read-only image is being opened with BDRV_O_RDWR. By not failing
for Linux block devices, the guest boots up but every write produces an
I/O error.
This patch checks whether the block device is read-only so that Linux
block devices behave like regular files.
Reported-by: Sibiao Luo <s...@redhat.com>
Suggested-by: Paolo Bonzini <pbonz...@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
Reviewed-by: Markus Armbruster <arm...@redhat.com>
Signed-off-by: Kevin Wolf <kw...@redhat.com>
---
block/raw-posix.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 48 insertions(+), 1 deletion(-)
diff --git a/block/raw-posix.c b/block/raw-posix.c
index 8b6b926..4dfdf98 100644
--- a/block/raw-posix.c
+++ b/block/raw-posix.c
@@ -1257,9 +1257,43 @@ static int hdev_probe_device(const char *filename)
return 0;
}
+static int check_hdev_writable(BDRVRawState *s)
+{
+#if defined(BLKROGET)
+ /* Linux block devices can be configured "read-only" using blockdev(8).
+ * This is independent of device node permissions and therefore open(2)
+ * with O_RDWR succeeds. Actual writes fail with EPERM.
+ *
+ * bdrv_open() is supposed to fail if the disk is read-only. Explicitly
+ * check for read-only block devices so that Linux block devices behave
+ * properly.
+ */
+ struct stat st;
+ int readonly = 0;
+
+ if (fstat(s->fd, &st)) {
+ return -errno;
+ }
+
+ if (!S_ISBLK(st.st_mode)) {
+ return 0;
+ }
+
+ if (ioctl(s->fd, BLKROGET, &readonly) < 0) {
+ return -errno;
+ }
+
+ if (readonly) {
+ return -EACCES;
+ }
+#endif /* defined(BLKROGET) */
+ return 0;
+}
+
static int hdev_open(BlockDriverState *bs, const char *filename, int flags)
{
BDRVRawState *s = bs->opaque;
+ int ret;
#if defined(__APPLE__) && defined(__MACH__)
if (strstart(filename, "/dev/cdrom", NULL)) {
@@ -1300,7 +1334,20 @@ static int hdev_open(BlockDriverState *bs, const char
*filename, int flags)
}
#endif
- return raw_open_common(bs, filename, flags, 0);
+ ret = raw_open_common(bs, filename, flags, 0);
+ if (ret < 0) {
+ return ret;
+ }
+
+ if (flags & BDRV_O_RDWR) {
+ ret = check_hdev_writable(s);
+ if (ret < 0) {
+ raw_close(bs);
+ return ret;
+ }
+ }
+
+ return ret;
}
#if defined(__linux__)
--
1.8.1.2
