| Message ID | 1359986911-5444-1-git-send-email-stefanha@redhat.com |
|---|---|
| State | New |
| Headers | show |
Stefan Hajnoczi <stefanha@redhat.com> writes: > LVM volumes can be set read-only with "lvchange --permission r > <volume>". The device node permissions remain unchanged so the volume > can still be opened O_RDWR. Actual writes will fail. Are you sure it's just LVM? blockdev(8) suggests it's a more general issue. Check out --setro there. > 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 LVM volumes, the guest boots up but every write produces an I/O > error. > > This patch checks whether the block device is read-only so that LVM > volumes behave like regular files. > > Reported-by: Sibiao Luo <sluo@redhat.com> > Suggested-by: Paolo Bonzini <pbonzini@redhat.com> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Patch looks good to me.
On Mon, Feb 04, 2013 at 03:49:10PM +0100, Markus Armbruster wrote: > Stefan Hajnoczi <stefanha@redhat.com> writes: > > > LVM volumes can be set read-only with "lvchange --permission r > > <volume>". The device node permissions remain unchanged so the volume > > can still be opened O_RDWR. Actual writes will fail. > > Are you sure it's just LVM? blockdev(8) suggests it's a more general > issue. Check out --setro there. You are right. I tested "blockdev --setro" and got the same behavior. I'll resend with comments indicating Linux block devices in general, instead of just LVM. Stefan
diff --git a/block/raw-posix.c b/block/raw-posix.c index 8b6b926..2ab4da4 100644 --- a/block/raw-posix.c +++ b/block/raw-posix.c @@ -1257,9 +1257,42 @@ static int hdev_probe_device(const char *filename) return 0; } +static int check_hdev_writable(BDRVRawState *s) +{ +#if defined(BLKROGET) + /* Linux LVM volumes can be configured "read-only" using lvchange(8). This + * does not change permissions on the device node 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 LVM volumes 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 +1333,20 @@ 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__)
LVM volumes can be set read-only with "lvchange --permission r <volume>". The device node permissions remain unchanged so the volume can still be opened O_RDWR. 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 LVM volumes, the guest boots up but every write produces an I/O error. This patch checks whether the block device is read-only so that LVM volumes behave like regular files. Reported-by: Sibiao Luo <sluo@redhat.com> Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> --- block/raw-posix.c | 48 +++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 47 insertions(+), 1 deletion(-)