linux-kernel - Re: linux-3.14 nfsd regression

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Date:	Thu, 03 Apr 2014 13:51:06 -0400
From:	Mark Lord <mlord@...ox.com>
To:	"J. Bruce Fields" <bfields@...hat.com>
CC:	Albert Fluegel <af@....de>, linux-nfs@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: linux-3.14 nfsd regression

On 14-04-03 01:16 PM, J. Bruce Fields wrote:
> On Thu, Apr 03, 2014 at 12:33:55PM -0400, Mark Lord wrote:
>> This commit from linux-3.14 breaks our NFS-root clients here:
>>
>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=6e14b46b91fee8a049b0940333ce13a820beaaa5
>>
>>
>> - *p++ = htonl((u32) stat->mode);
>> + *p++ = htonl((u32) (stat->mode & S_IALLUGO));
>>
>>
>> Reverting the one-liner above (on the server) fixes it for us,
>> as does reverting back to linux-3.13.8 on the server.
>>
>> The NFS-root clients are on PowerPC (big-endian) architecture,
>> running linux-3.12.16. The NFS server is on an Intel PC running linux-3.14.
>>
>> ACL is completely disabled on server and client,
>> and we're using NFSv2/v3.  No support for v4.
>>
>> I instrumented the function to see what other bits were being cleared
>> by the (stat->mode & S_IALLUGO) masking.  The results are attached.
> 
> Hm, it sounds like a bug in the client if it's depending on those high
> bits.

But only for mounting / starting up from the nfsroot, it seems.
I wonder if there's an unusual code path for that in there?
The regular stuff looks mostly fine:

        p = xdr_decode_ftype3(p, &fmode);
        fattr->mode = (be32_to_cpup(p++) & ~S_IFMT) | fmode;

Except perhaps that second line ought to use the same mask
as the server side is using, just in case there are some other
stray high (higher than S_IFMT) bits in there now/someday.

> The original behavior was in practice harmless and changing it broke
> something, so I think we should definitely just revert this patch.

Yup.  Who?

> But the client may need fixing too.

Probably a good thing in the longer term, for better compatibility
with non-Linux servers.  But we'll still have to keep the revert
on the server (nfsd) code for backward compatibility, I think.

Cheers


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