On Mon, 2011-08-01 at 07:19 -0400, Christoph Hellwig wrote: > On Mon, Aug 01, 2011 at 12:10:12PM +0100, Sachin Prabhu wrote: > > Do not allow multiple mounts on same mountpoint when using -o noac > > The patch content really doesn't seem to match the subject line and > most of the content. > > > However when using the option 'noac', the user is able to mount the same > > share on the same mountpoint multiple times. This happens because a > > share mounted with the noac option is automatically assigned the 'sync' > > flag MS_SYNCHRONOUS in nfs_initialise_sb(). This flag is set after the > > check for already existing superblocks is done in sget(). The check for > > the mount flags in nfs_compare_mount_options() does not take into > > account the 'sync' flag applied later on in the code path. This means > > that when using 'noac', a new superblock structure is assigned for every > > new mount of the same share and multiple shares on the same mountpoint > > are allowed. > > > > ie. > > # mount -onoac localhost:/nfsv3 /mnt > > can be run multiple times. > > > The patch checks for noac and assigns the sync flag before sget() is > > called to obtain an already existing superblock structure. > > That's a fine fix, but the the whole patch subject and description > focusses on a side effect rather than the underlying real problem. > > The underlying issue is that youwant to share superblocks when > having multiple mounts with -o noac, which requires assigning the > sync flag ealier. > > The check in do_add_mount simply prevents mounts with the same > superblock and root dentry at the same mountpoint, so it will fail for > any two nfs mounts that might be from the same server but do not share > the superblock due to different options. I'm actually not sure we want > to prevent that in general, and I can't see how it could be done easily. > Yes. That is essentially what the problem is. The problem was reported by a user who noticed that passing the 'noac' mount option allowed them to repeatedly mount the same share on the same mount point. I just used the same perspective when reporting the issue upstream. Sachin Prabhu -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
