linux-kernel - Re: [PATCH] overlayfs: ignore empty NFSv4 ACLs in ext4 upperdir

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Date:   Tue, 6 Dec 2016 00:24:30 +0100
From:   Andreas Grünbacher <andreas.gruenbacher@...il.com>
To:     Patrick Plagwitz <Patrick_Plagwitz@....de>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        "linux-unionfs@...r.kernel.org" <linux-unionfs@...r.kernel.org>,
        Linux NFS list <linux-nfs@...r.kernel.org>,
        Linux FS-devel Mailing List <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andreas Gruenbacher <agruenba@...hat.com>
Subject: Re: [PATCH] overlayfs: ignore empty NFSv4 ACLs in ext4 upperdir

2016-12-06 0:19 GMT+01:00 Andreas Grünbacher <andreas.gruenbacher@...il.com>:
> 2016-12-05 23:58 GMT+01:00 Patrick Plagwitz <Patrick_Plagwitz@....de>:
>> On 12/05/2016 08:37 PM, Andreas Grünbacher wrote:
>>> 2016-12-05 17:25 GMT+01:00 J. Bruce Fields <bfields@...ldses.org>:
>>>> On Mon, Dec 05, 2016 at 04:36:03PM +0100, Miklos Szeredi wrote:
>>>>> On Mon, Dec 5, 2016 at 4:19 PM, J. Bruce Fields <bfields@...ldses.org> wrote:
>>>>>>> Can NFS people comment on this?  Where does the nfs4_acl come from?
>>>>>>
>>>>>> This is the interface the NFS client provides for applications to modify
>>>>>> NFSv4 ACLs on servers that support them.
>>>>>
>>>>> Fine, but why are we seeing this xattr on exports where no xattrs are
>>>>> set on the exported fs?
>>>>
>>>> I don't know.  I took another look at the original patch and don't see
>>>> any details on the server setup: which server is it (knfsd, ganesha,
>>>> netapp, ...)?  How is it configured?
>>>>
>>>>>>> What can overlayfs do if it's a non-empty ACL?
>>>>>>
>>>>>> As little as possible.  You can't copy it up, can you?  So any attempt
>>>>>> to support it is going to be incomplete.
>>>>>
>>>>> Right.
>>>>>
>>>>>>
>>>>>>> Does knfsd translate posix ACL into NFS acl?  If so, we can translate
>>>>>>> back.  Should we do a generic POSIX<->NFS acl translator?
>>>>>>
>>>>>> knsd does translate between POSIX and NFSv4 ACLs.  It's a complicated
>>>>>
>>>>> This does explain the nfs4_acl xattr on the client.  Question: if it's
>>>>> empty, why have it at all?
>>>>
>>>> I'm honestly not sure what's going on there.  I'd be curious to see a
>>>> network trace if possible.
>>>
>>> I do see "system.nfs4_acl" attributes on knfsd exported filesystems
>>> that support POSIX ACLs (for ext4: "mount -o acl"). For exported
>>> filesystem that don't support POSIX ACLs (ext4: mount -o noacl), that
>>> attribute is missing. The attribute shouldn't be empty though; when
>>> the file has no real ACL, "system.nfs4_acl" represents the file mode
>>> permissions. The "system.nfs4_acl" attribute exposes the information
>>> on the wire; there is no resonable way to translate that into an ACL
>>> on another filesystem, really.
>>>
>>> Patrick, what does 'getfattr -m- -d /nfs/file' give you?
>>>
>> getfattr -m - -d nfs/folder -e text gives
>>
>> # file: nfs/folder/
>> system.nfs4_acl="\000\000\000^C\000\000\000\000\000\000\000\000\000^V^A<E7>\000\000\000^FOWNER@\000\000\000\000\000\000\000\000\000\000\000^R\000<A1>\000\000\000^FGROUP@\000\000\000\000\000\000\000\000\000\000\000^R\000<A1>\000\000\000
>>     EVERYONE@\000\000"
>>
>> Those are 80 bytes. I checked again and vfs_getxattr indeed returns size=80.
>> It just looked empty because the first byte is 0... Ok, so nfs4_acl is not
>> empty after all and checking *value == 0 does not tell if there are actually
>> ACLs present or not, sorry for the confusion.
>>
>> You are right, when I mount the exported fs with noacl the problem goes away.
>> You already helped me there, thanks.
>>
>> Still, I think there should be a way to copy up files that actually have no
>> ACLs since acl is often the default for ext4 mounts and giving an "Operation
>> not supported" for random open(2)s is not a very good way to convey what's
>> going on.
>
> It's not hard to come up with a heuristic that determines if a
> system.nfs4_acl value is equivalent to a file mode, and to ignore the
> attribute in that case. (The file mode is transmitted in its own
> attribute already, so actually converting .) That way, overlayfs could
> still fail copying up files that have an actual ACL. It's still an
> ugly hack ...

Actually, that kind of heuristic would make sense in the NFS client
which could then hide the "system.nfs4_acl" attribute.

Andreas