linux-kernel - Re: For review: open_by_name

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Message-ID: <53295ED0.7070304@gmail.com>
Date:	Wed, 19 Mar 2014 10:09:36 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	NeilBrown <neilb@...e.de>
CC:	mtk.manpages@...il.com,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Andreas Dilger <adilger@...ger.ca>,
	Christoph Hellwig <hch@...radead.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: For review: open_by_name_at(2) man page [v2]

[CC =+ Al Viro]

Hi Neil,

On 03/19/2014 05:13 AM, NeilBrown wrote:
> On Tue, 18 Mar 2014 13:55:15 +0100 "Michael Kerrisk (man-pages)"
> <mtk.manpages@...il.com> wrote:
> 
>> Hi Aneesh, (and others)
>>
>> After integrating review comments from NeilBown and Christoph Hellwig,
>> here is draft 2 of a man page I've written for name_to_handle_at(2) and
>> open_by_name_at(2). Especially thanks to Neil's comments, several parts
>> of the page underwent a substantial rewrite. Would you be willing to 
>> review it please, and let me know of any corrections/improvements?

[various typos you reported fixed now.]

>> .TP
>> .B AT_EMPTY_PATH
>> Allow
>> .I pathname
>> to be an empty string.
>> See above.
>> (which may have been obtained using the
>> .BR open (2)
>> .B O_PATH
>> flag).
> 
> What "may have been obtained" ??

Crufty text. gone now.

>> The
>> .I flags
>> argument
>> is as for
>> .BR open (2).
>> .\" FIXME: Confirm that the following is intended behavior.
>> .\"        (It certainly seems to be the behavior, from experimenting.)
>> If
>> .I handle
>> refers to a symbolic link, the caller must specify the
>> .B O_PATH
>> flag, and the symbolic link is not dereferenced (the
>> .B O_NOFOLLOW
>> flag, if specified, is ignored).
> 
> It certainly sounds like reasonable behaviour.  I cannot comment on intention
> though.
> Are you bothered that O_PATH is needed for symlinks?  

No.

> An fd on a symlink is a
> sufficiently unusual thing that it seems reasonable for a programmer to
> explicitly say they are expecting one.

I think the point is this: If you have a file handle for a symlink, then
you can't follow the symlink, which is why you must specify O_PATH and
O_NOFOLLOW becomes irrelevant. I'm curious about the rationale though.
I suspect it's something like: the process receiving the handle doesn't
have enough information for the symlink to be interpreted, I think because
it can;t reliably determine what directory the link lives in.
Possibly Al Viro or Aneesh can confirm.

>> In the event of an error, both system calls return \-1 and set
>> .I errno
>> to indicate the cause of the error.
>> .SH ERRORS
>> .BR name_to_handle_at ()
>> and
>> .BR open_by_handle_at ()
>> can fail for the same errors as
>> .BR openat (2).
>> In addition, they can fail with the errors noted below.
> 
> Should you mention EFAULT if mount_id or handle are not valid pointers?

Done.

>> Not all filesystem types support the translation of pathnames to
>> file handles.
>> .\" FIXME NeilBrown noted:
>> .\"    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. open_by_name_at given this (or any) filehandle
>> .\"    will fail with ESTALE.
>> .\" However, on /proc and /sys, at least, name_to_handle_at() fails with
>> .\" EOPNOTSUPP. Are there really filesystems that can deliver ESTALE (the
>> .\" same error as for an invalid file handle) in the above circumstances?
> 
> This is all wrong - discard it :-)

Yup. Gone 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/
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