linux-kernel - Re: Regression in 5.1.20: Reading long directory fails

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Message-Id: <3B2EEB3C-3305-4A50-A55B-51093A985284@oracle.com>
Date:   Sun, 8 Sep 2019 11:48:58 -0400
From:   Chuck Lever <chuck.lever@...cle.com>
To:     Trond Myklebust <trondmy@...merspace.com>
Cc:     "bcodding@...hat.com" <bcodding@...hat.com>,
        "tibbs@...h.uh.edu" <tibbs@...h.uh.edu>,
        Bruce Fields <bfields@...ldses.org>,
        "linux@...m.de" <linux@...m.de>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "km@...all.com" <km@...all.com>
Subject: Re: Regression in 5.1.20: Reading long directory fails



> On Sep 8, 2019, at 11:19 AM, Trond Myklebust <trondmy@...merspace.com> wrote:
> 
> On Sun, 2019-09-08 at 07:39 -0400, Benjamin Coddington wrote:
>> On 6 Sep 2019, at 16:50, Chuck Lever wrote:
>> 
>>>> On Sep 6, 2019, at 4:47 PM, Jason L Tibbitts III <
>>>> tibbs@...h.uh.edu> 
>>>> wrote:
>>>> 
>>>>>>>>> "JBF" == J Bruce Fields <bfields@...ldses.org> writes:
>>>> 
>>>> JBF> Those readdir changes were client-side, right?  Based on
>>>> that 
>>>> I'd
>>>> JBF> been assuming a client bug, but maybe it'd be worth getting
>>>> a 
>>>> full
>>>> JBF> packet capture of the readdir reply to make sure it's legit.
>>>> 
>>>> I have been working with bcodding on IRC for the past couple of
>>>> days 
>>>> on
>>>> this.  Fortunately I was able to come up with way to fill up a 
>>>> directory
>>>> in such a way that it will fail with certainty and as a bonus
>>>> doesn't
>>>> include any user data so I can feel OK about sharing packet
>>>> captures. 
>>>> I
>>>> have a capture alongside a kernel trace of the problematic
>>>> operation 
>>>> in
>>>> https://www.math.uh.edu/~tibbs/nfs/.  Not that I can
>>>> particularly 
>>>> tell
>>>> anything useful from that, but bcodding says that it seems to
>>>> point 
>>>> to
>>>> some issue in sunrpc.
>>>> 
>>>> And because I can easily reproduce this and I was able to do a 
>>>> bisect:
>>>> 
>>>> 2c94b8eca1a26cd46010d6e73a23da5f2e93a19d is the first bad commit
>>>> commit 2c94b8eca1a26cd46010d6e73a23da5f2e93a19d
>>>> Author: Chuck Lever <chuck.lever@...cle.com>
>>>> Date:   Mon Feb 11 11:25:41 2019 -0500
>>>> 
>>>>   SUNRPC: Use au_rslack when computing reply buffer size
>>>> 
>>>>   au_rslack is significantly smaller than (au_cslack << 2).
>>>> Using
>>>>   that value results in smaller receive buffers. In some cases
>>>> this
>>>>   eliminates an extra segment in Reply chunks (RPC/RDMA).
>>>> 
>>>>   Signed-off-by: Chuck Lever <chuck.lever@...cle.com>
>>>>   Signed-off-by: Anna Schumaker <Anna.Schumaker@...app.com>
>>>> 
>>>> :040000 040000 d4d1ce2fbe0035c5bd9df976b8c448df85dcb505 
>>>> 7011a792dfe72ff9cd70d66e45d353f3d7817e3e M      net
>>>> 
>>>> But of course, I can't say whether this is the actual bad commit
>>>> or
>>>> whether it just introduced a behavior change which alters the 
>>>> conditions
>>>> under which the problem appears.
>>> 
>>> The first place I'd start looking is the XDR constants at the head
>>> of 
>>> fs/nfs/nfs4xdr.c
>>> having to do with READDIR.
>>> 
>>> The report of behavior changes with the use of krb5p also makes
>>> this 
>>> commit plausible.
>> 
>> After sprinkling the printk's, we're coming up one word short in the 
>> receive
>> buffer.  I think we're not accounting for the xdr pad of buf->pages
>> for 
>> NFS4
>> readdir -- but I need to check the RFCs.  Anyone know if v4 READDIR 
>> results
>> have to be aligned?
>> 
>> Also need to check just why krb5i is the only auth that cares..
>> 
> 
> I'm not seeing that. If you look at commit 02ef04e432ba, you'll see
> that Chuck did add a 'padding term' to decode_readdir_maxsz in the
> NFSv4 case.
> The other thing to remember is that a readdir 'dirlist4' entry is
> always word aligned (irrespective of the length of the filename), so
> there is no padding that needs to be taken into account.
> 
> I think we probably rather want to look at how auth->au_ralign is being
> calculated for the case of krb5i. I'm really not understanding why
> auth->au_ralign should not take into account the presence of the mic.
> Chuck?

I'm looking at gss_unwrap_resp_integ():

1971         auth->au_rslack = auth->au_verfsize + 2 + 1 + XDR_QUADLEN(mic.len);
1972         auth->au_ralign = auth->au_verfsize + 2;

au_ralign now sets the alignment of the _start_ of the RPC message body.
The MIC comes _after_ the RPC message body for krb5i.

If Ben is off by one quad, that's not the MIC, which is typically 32 octets,
isn't it?

Maybe some variable-length data item in the returned file attributes is missing
an XDR pad.

--
Chuck Lever