Re: For review: open_by_name_at(2) man page

[NeilBrown]

On Tue, 18 Mar 2014 13:37:15 +0100 "Michael Kerrisk (man-pages)"
<mtk.manpages@xxxxxxxxx> wrote:

> On 03/18/2014 10:43 AM, Christoph Hellwig wrote:
> > On Tue, Mar 18, 2014 at 09:00:07AM +1100, NeilBrown wrote:
> >> ESTALE is also returned if the filesystem does not support file-handle ->
> >> file mappings.
> >> On filesystems which don't provide export_operations (/sys /proc ubifs
> >> romfs cramfs nfs coda ... several others) name_to_handle_at will produce a
> >> generic handle using the 32 bit inode and 32 bit i_generation.
> > 
> > Do we?  Seems like the code is erroring out early if there are no
> > export_ops?
> 
> It appears to me that Neil's statement isn't correct, at least for /proc
> and /sys (see my other mail, to Neil). I'm unsure about whether it is true
> for some of those other FSes thought.


Indeed, I was wrong.

I was looking at

int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
			     int *max_len, struct inode *parent)
{
	const struct export_operations *nop = inode->i_sb->s_export_op;

	if (nop && nop->encode_fh)
		return nop->encode_fh(inode, fid->raw, max_len, parent);

	return export_encode_fh(inode, fid, max_len, parent);
}


which uses a default if there is no 'nop'.

However do_sys_name_to_handle() contains

	if (!path->dentry->d_sb->s_export_op ||
	    !path->dentry->d_sb->s_export_op->fh_to_dentry)
		return -EOPNOTSUPP;

long before export_encode_inode_fh() gets called.  So the default isn't used.

I would have thought that exportfs_encode_inode_fh would never get called if
there were no s_export_op pointer - certainly name_to_handle_at and nfsd
would never call it in that case.
However it seems that

    This routine will be used to generate a file handle in fdinfo output for
    inotify subsystem, where if no s_export_op present the general
    export_encode_fh should be used.  Thus add a test if s_export_op present
    inside exportfs_encode_fh itself.

according to

commit ab49bdecc3ebb46ab661f5f05d5c5ea9606406c6
Author: Cyrill Gorcunov <gorcunov@xxxxxxxxxx>
Date:   Mon Dec 17 16:05:06 2012 -0800


I guess that means that you can extract filehandles from /proc/self/fdinfo/$FD
when $FD is an inotify fd which is watching the particular file.....  I
wouldn't have expected that, but maybe it is a good idea.

So yes: if the filesystem doesn't support filehandles you get EOPNOTSUPP.
So if you get ESTALE from open_by_handle_at(), then it really is a stale
handle.  Sorry for the confusion.

NeilBrown

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