block: Require aligned image size to avoid assertion failure

Unaligned requests will automatically be aligned to bl.request_alignment
and we can't extend write requests to access space beyond the end of the
image without resizing the image, so if we have the WRITE permission,
but not the RESIZE one, it's required that the image size is aligned.

Failing to meet this requirement could cause assertion failures like
this if RESIZE permissions weren't requested:

qemu-img: block/io.c:1910: bdrv_co_write_req_prepare: Assertion `end_sector <= bs->total_sectors || child->perm & BLK_PERM_RESIZE' failed.

This was e.g. triggered by qemu-img converting to a target image with 4k
request alignment when the image was only aligned to 512 bytes, but not
to 4k.

Turn this into a graceful error in bdrv_check_perm() so that WRITE
without RESIZE can only be taken if the image size is aligned. If a user
holds both permissions and drops only RESIZE, the function will return
an error, but bdrv_child_try_set_perm() will ignore the failure silently
if permissions are only requested to be relaxed and just keep both
permissions while returning success.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Message-Id: <20200716142601.111237-2-kwolf@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -2025,6 +2025,22 @@ static int bdrv_check_perm(BlockDriverState *bs, BlockReopenQueue *q,
         return -EPERM;
     }
 
+    /*
+     * Unaligned requests will automatically be aligned to bl.request_alignment
+     * and without RESIZE we can't extend requests to write to space beyond the
+     * end of the image, so it's required that the image size is aligned.
+     */
+    if ((cumulative_perms & (BLK_PERM_WRITE | BLK_PERM_WRITE_UNCHANGED)) &&
+        !(cumulative_perms & BLK_PERM_RESIZE))
+    {
+        if ((bs->total_sectors * BDRV_SECTOR_SIZE) % bs->bl.request_alignment) {
+            error_setg(errp, "Cannot get 'write' permission without 'resize': "
+                             "Image size is not a multiple of request "
+                             "alignment");
+            return -EPERM;
+        }
+    }
+
     /* Check this node */
     if (!drv) {
         return 0;