Re: [PATCH RFC] NFSD: fix cannot umounting mount points under pseudo root

[Kinglong Mee <kinglongmee@gmail.com>]

Cc Steve, Viro,

On 5/1/2015 5:36 AM, J. Bruce Fields wrote:
> On Thu, Apr 30, 2015 at 07:52:25AM +1000, NeilBrown wrote:
>> On Wed, 29 Apr 2015 15:19:34 -0400 "J. Bruce Fields" <bfields@fieldses.org>
>> wrote:
>>> Maybe drop the locking from nfsd_buffered_readdir and *just* take the
>>> i_mutex around lookup_one_len(), if that's the only place we need it?

As description in other thread, before the upcall to rpc.mountd,
nfsd have call lookup_one_len() for the file, but why rpc.mountd
also blocked in lookup ?

There is a bug in rpc.mountd when checking sub-directory, 
it sets bad patch length for child.

If parent if "/nfs/xfs" and child is "/nfs/test", the child name
will be truncated to "/nfs/tes" for strlen(parent), "/nfs/test"
have exist in kernel's cache for the lookup_one_len(), but
"/nfs/tes" is a bad path, which needs lookup_slow(), so blocked.

static int is_subdirectory(char *child, char *parent)
{
        /* Check is child is strictly a subdirectory of
         * parent or a more distant descendant.
         */
        size_t l = strlen(parent);

        if (strcmp(parent, "/") == 0 && child[1] != 0)
                return 1;

        return (same_path(child, parent, l) && child[l] == '/');
}

The following path makes a correct path, not a truncated path.
Have be tested, everything is OK.

thanks,
Kinglong Mee

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