linux-kernel - Re: tmpfs support of xattrs?

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Date:	Mon, 27 Oct 2008 13:14:22 -0700
From:	Rob MacKinnon <c4blem0nkey@...il.com>
To:	Andreas Gruenbacher <agruen@...e.de>, hpa@...or.com,
	hugh@...itas.com, agruen@...e.de
CC:	sds@...ho.nsa.gov, linux-kernel@...r.kernel.org,
	rob.mackinnon@...il.com
Subject: Re: tmpfs support of xattrs?

Andreas Gruenbacher wrote:
> On Sunday, 26 October 2008 13:26:35 Hugh Dickins wrote:
>> On Sat, 25 Oct 2008, Rob MacKinnon wrote:
>>> The background: So during the initial configuration of a box I enabled
>>> the xattr flags for ext3 and options of xattr in coreutils, and at the
>>> time didn't realize that I'd hit a snag that would finally annoy me
>>> enough after a month of getting a non-fatal error messages from cp "cp:
>>> listing attributes of `/dev/null`: Invalid argument" to spend half a day
>>> researching the cause and a potential solution.
>>>
>>> Setup: udev mounts a tmpfs to /dev then fills it with device nodes.
>>> Problem: the resulting tmpfs has no xattr support.
>>> Therefore: Tmpfs without xattrs, and coreutils  and everywhere else with
>>> xattr support, cp freaks.
>>>
>>> Is there sometime in the forseable future when the tmpfs module will
>>> support for xattrs in the stable branch, or should I "holler at the
>>> maintainers of coreutils to fix their broken code in cp". Even better
>>> (and I like this option the most) a little of both?
>> I've not seen "cp: listing attributes of `/dev/null': Invalid argument"
>> messages (or.. do I have a dim recollection of them once upon a time?).
>> I would certainly get irritated by them if I did, and want to fix them.
>> I tried "cp /dev/null temp" on a few distros just now but not seen it.
> 
> As Hugh pointed out already, tmpfs does support xattrs since quite a while, so 
> enabling that should fix the symptoms. Independent of that, cp must of course 
> work correctly on filesystems that don't have xattrs. Those filesystems are 
> not supposed to return EINVAL, though.
> 
> Could you please run cp under strace to show us what's happening at the 
> syscall level (and perhaps under ltrace in addition to see the library level 
> as well)?
> 
> If the error happens at the syscall level (*listxattr), then which kernel is 
> it exactly, and where can I find its sources?
> 

Thanks Andreas, Hugh & HPA

First off, thanks for looking into this.  After googling around, hitting
forums and such and not getting any answers, this list seemed the best
place to come to ask.

Now onward to business...I can say that I too was under the general
understanding that xattr for tmpfs was added at 2.6.10...and was still
currently supported.  Unfortunately I already have the config flags set
(as seen below, for a full config set attached) so there is a continued
confusion in my mind why this is still plaguing me.

I've had several friends compare what was going on, and of them they
were all running rhel and cent (most with no udev support so the
comparison didn't help at all).

I'll start out with distro info and the like...
`uname -a`="Linux shin-yuko 2.6.26-gentoo-r2 #1 SMP PREEMPT Wed Oct 22
16:31:40 PDT 2008 i686 Intel(R) Pentium(R) 4 CPU 2.80GHz GenuineIntel
GNU/Linux"

coreutils-version="coreutils-6.12-r2"

My current config does have the following set:
	CONFIG_TMPFS=y
	CONFIG_TMPFS_POSIX_ACL=y
	CONFIG_SHMEM=y

Output from `strace /bin/cp /dev/null /root/temp`= #see attachment
Output from `ltrace -CS /bin/cp /dev/null /root/temp`= #see attachment

Output from `attr -l /dev/null`=Segfault (hmmm...lemme look at a strace
and ltrace of that too)

`strace attr -l /dev/null`= #see attachment
`ltrace -CS /bin/attr -l /dev/null`= #see attachment

Kernel source can be attained from:
http://distfiles.gentoo.org/distfiles/linux-2.6.26.tar.bz2

Patches:
http://sources.gentoo.org/viewcvs.py/linux-patches/genpatches-2.6/trunk/2.6.26/

I know this is silly but, setfattr and getfattr obviously don't work
either...just thought it was worth mentioning.

-- Rob

View attachment "strace attr.txt" of type "text/plain" (2230 bytes)

View attachment "strace cp (with xattr support).txt" of type "text/plain" (3722 bytes)

View attachment "ltrace cp (with xattr support).txt" of type "text/plain" (11075 bytes)

View attachment "ltrace attr.txt" of type "text/plain" (4009 bytes)

View attachment "kernel-config-x86-2.6.26-gentoo-r2" of type "text/plain" (66697 bytes)