As mentioned by Bruce, GlusterFS doesn't have owner-override rule except
for setattr.
I did few experiments to check why this test case passes on plain
glusterfs fuse mount & NFS-Ganesha but fails with kernel-NFS.
NFS-Ganesha (for most of the FSALs) seem to be passing the actual
request credentials to the back-end filesystem only for CREATE(-like)
and UNLINK fops. For all the remaining fops, it does the access check at
its end and then perform the operation with root credentials. That's the
reason WRITE succeeded in your case as NFS-Ganesha (like kernel-NFS)
skipped the access check if the request caller_uid proved to be the
file's owner.
In case of native GlusterFS FUSE mount, there is no OPEN fop involved.
WRITE is performed on the fd returned by CREATE. And strangely GlusterFS
seem to be doing certain access checks only during OPEN but not for
WRITE (this seems like a bug and probably needs to be fixed in Gluster).
Thanks,
Soumya
On 12/14/2015 10:27 PM, Omar Walid Llorente wrote:
Thank you Bruce, others, for the responses. I send attached a complete
capture of the issue, including the glusterfs transactions.
Hope this helps to clear where may it be...
Omar
El 10/12/15 a las 15:44, J. Bruce Fields escribió:
On Thu, Dec 10, 2015 at 05:59:33PM +0530, Soumya Koduri wrote:
On 12/10/2015 04:02 PM, Omar Walid Llorente wrote:
Hi, Jeff, Bruce, finally I got some time to get the capture of the nfs
packets (you can find them in attached file nfs-problem-nks.pcap.zip).
Sorry for being so late.
What I did was the following:
1st) Create the RO file:
cdc@l056:~/prueba-git$ rm -f kk.txt 444.txt; echo "prueba" > 444.txt;
chmod 444 444.txt;
2nd) Init the capture:
root@l056:~# tcpdump -i eth2 -w /tmp/nfs.pcap -s 512 port 2049
tcpdump: listening on eth2, link-type EN10MB (Ethernet), capture size
512 bytes
GlusterFS protocol is added to wireshark from version 1.8.0 [1]. It
may be helpful to see what GlusterFS operations are being processed
as part of NFS WRITE call (which has failed in this case).
Could you please try taking the packet trace on the machine where
NFS server is running (without filtering out based on the port
number).
Also I tried out the same test on Fedora22 machine, but haven't run
into any issue. What are the fuse mount options you have used to
mount gluster volume?
Oh, I think this is a simple problem (but maybe hard to fix). The
capture shows NFSv3 traffic like:
CREATE -> OK
SETATTR (mode set to 0400) -> OK
WRITE -> NFS3ERR_ACCES
That write would succeed locally (because the mode doesn't matter to a
local application that already holds the file open). It would fail over
NFSv3, which doesn't know about the open--except that there's a hack for
this case: NFSv3 servers allow IO operations to ignore the mode, if the
operation comes from the owner of the file. NFSv3 clients are then
careful to perform necessary access checks on open to ensure that this
owner-override rule doesn't grant too many permissions.
That allows NFSv3 applications to see behavior that's mostly like a
local filesystem, without opening much of a security hole (since the
owner could always chmod anyway).
So, knfsd is making this special exception--but gluster (which I believe
it's exporting in this case, via fuse?)--probably doesn't.... I'm not
sure what you can do about that.
--b.
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