Re: For review: open_by_name_at(2) man page

["Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>]

Hi Neil,

On 03/18/2014 11:24 PM, NeilBrown wrote:
> On Tue, 18 Mar 2014 13:37:15 +0100 "Michael Kerrisk (man-pages)"
> <mtk.manpages@gmail.com> 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.

Okay.

> 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@openvz.org>
> 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.

Yes, it does--I tested it, and it works! I was unaware of this feature,
though I'm not sure that I'll add anything to a man page just yet.

> 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.

Yup, I've updated the page now.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/