Re: For review: open_by_name_at(2) man page

From: Michael Kerrisk (man-pages)
Date: Wed Mar 19 2014 - 05:11:03 EST

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@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.

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.



Michael Kerrisk
Linux man-pages maintainer;
Linux/UNIX System Programming Training:
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