On 01/18/2010 01:59 PM, Al Viro wrote:
> On Mon, Jan 18, 2010 at 12:21:09PM +0800, Ian Kent wrote:
>
>> In that case we may find an autofs mount that has something mounted on
>> top of it and user space wants to know the super of the covering mount.
>>
>> If there is something mounted on top of it user space needs to know if
>> it is another autofs file system or some other type of file system. So
>> if the nameidata path, located by autofs_dev_ioctl_find_super(), is not
>> the top (or bottom, depending on the terminology you prefer) then we
>> need to follow the mount and return the magic of the thing mounted on
>> top of it.
>
> IDGI. What you are doing there is
> if (path.mnt->mnt_mountpoint != path.mnt->mnt_root) {
> if (follow_down(&path))
> magic = path.mnt->mnt_sb->s_magic;
> }
> and I don't think it means what you think it means. Just what is that
> mnt_mountpoint check about? Before that point we'd found the autofs
> vfsmount M that
> 1) M is mounted on <name>
> 2) M->mnt_sb has the right s_dev
> 3) M is the closest one to root in mount tree out of vfsmounts
> satisfying (1) and (2)
> Now we check that
> 4) the mountpoint M is attached to has dentry different from
> M->mnt_root. That's an interesting thing to check, seeing that the
> only way to get it false is to have mount --bind name name, with name
> not being the mountpoint before that. And M being the result of
> that mount --bind.
> 5) something is mounted on top of root of M.
>
> Then we proceed to return the s_magic of that something. For one thing,
> if there *are* several vfsmounts satisfying (1,2), which one do we really
> want? For another, what's the intent of (4)? It looks very odd; what's
> really being checked there?
The code here has changed a little from what I originally posted but,
assuming for now the functionality is equivalent, I can't see what the
point of that check is and now I can't remember if there was some odd
special case. follow_mount() should be sufficient. I'll fix this.
The possibility of more than one vfsmount being present is, as you say
possible, but it is not legal for autofs (and last time I checked I
concluded it wasn't possible for me to veto bind mount requests). Other
than bind mounts I'm struggling to think of a case where I would have
more than one autofs fs mount with the same s_dev.
>
> In another branch we have
> if (path.dentry->d_inode &&
> path.mnt->mnt_root == path.dentry) {
> err = 1;
> magic = path.dentry->d_inode->i_sb->s_magic;
> }
> and AFAICT, path.dentry->d_inode == NULL is impossible there. Besides,
> path.mnt->mnt_sb->s_magic would be simpler anyway (and evaluate to
> the same thing).
Yes, the dentry should always be positive here but let me think about it
a little more in case I'm missing something. And yes, using the vfsmount
super block pointer would be better. I'll fix these too.
Ian
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