Re: [PATCH v2 1/2] NFSD: Encode COMPOUND operation status on page boundaries

Chuck Lever
2024-12-25T15:54:29Z

On 12/24/24 6:19 PM, NeilBrown wrote:
> On Tue, 24 Dec 2024, cel@kernel.org wrote:
>> From: Chuck Lever <chuck.lever@oracle.com>
>>
>> J. David reports an odd corruption of a READDIR reply sent to a
>> FreeBSD client.
>>
>> xdr_reserve_space() has to do a special trick when the @nbytes value
>> requests more space than there is in the current page of the XDR
>> buffer.
>>
>> In that case, xdr_reserve_space() returns a pointer to the start of
>> the next page, and then the next call to xdr_reserve_space() invokes
>> __xdr_commit_encode() to copy enough of the data item back into the
>> previous page to make that data item contiguous across the page
>> boundary.
>>
>> But we need to be careful in the case where buffer space is reserved
>> early for a data item that will be inserted into the buffer later.
>>
>> One such caller, nfsd4_encode_operation(), reserves 8 bytes in the
>> encoding buffer for each COMPOUND operation. However, a READDIR
>> result can sometimes encode file names so that there are only 4
>> bytes left at the end of the current XDR buffer page (though plenty
>> of pages are left to handle the remaining encoding tasks).
>>
>> If a COMPOUND operation follows the READDIR result (say, a GETATTR),
>> then nfsd4_encode_operation() will reserve 8 bytes for the op number
>> (9) and the op status (usually NFS4_OK). In this weird case,
>> xdr_reserve_space() returns a pointer to byte zero of the next buffer
>> page, as it assumes the data item will be copied back into place (in
>> the previous page) on the next call to xdr_reserve_space().
>>
>> nfsd4_encode_operation() writes the op num into the buffer, then
>> saves the next 4-byte location for the op's status code. The next
>> xdr_reserve_space() call is part of GETATTR encoding, so the op num
>> gets copied back into the previous page, but the saved location for
>> the op status continues to point to the wrong spot in the current
>> XDR buffer page because __xdr_commit_encode() moved that data item.
>>
>> After GETATTR encoding is complete, nfsd4_encode_operation() writes
>> the op status over the first XDR data item in the GETATTR result.
>> The NFS4_OK status code (0) makes it look like there are zero items
>> in the GETATTR's attribute bitmask.
>>
>> The patch description of commit 2825a7f90753 ("nfsd4: allow encoding
>> across page boundaries") [2014] remarks that NFSD "can't handle a
>> new operation starting close to the end of a page." This bug appears
>> to be one reason for that remark.
>>
>> Reported-by: J David <j.david.lists@gmail.com>
>> Closes: https://lore.kernel.org/linux-nfs/3998d739-c042-46b4-8166-dbd6c5f0e804@oracle.com/T/#t
>> X-Cc: stable@vger.kernel.org
>> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
>> ---
>> fs/nfsd/nfs4xdr.c | 20 ++++++++++----------
>> 1 file changed, 10 insertions(+), 10 deletions(-)
>>
>> diff --git a/fs/nfsd/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
>> index 53fac037611c..15cd716e9d91 100644
>> --- a/fs/nfsd/nfs4xdr.c
>> +++ b/fs/nfsd/nfs4xdr.c
>> @@ -5760,15 +5760,14 @@ nfsd4_encode_operation(struct nfsd4_compoundres *resp, struct nfsd4_op *op)
>> struct nfs4_stateowner *so = resp->cstate.replay_owner;
>> struct svc_rqst *rqstp = resp->rqstp;
>> const struct nfsd4_operation *opdesc = op->opdesc;
>> - int post_err_offset;
>> + unsigned int op_status_offset;
>
> As most uses of "op_status_offset" add XDR_UNIT I'd be incline to keep
> the "post" offset.
>
> unsigned int op_status_offset, post_status_offset;

My thinking is that the "op_status_offset" value will be used on every
call of this function, but the "post_status_offset" cases are actually
exceedingly rare. There's no good reason to maintain that value
separately during every call.

>> nfsd4_enc encoder;
>> - __be32 *p;
>>
>> - p = xdr_reserve_space(xdr, 8);
>> - if (!p)
>> + if (xdr_stream_encode_u32(xdr, op->opnum) != XDR_UNIT)
>> + goto release;
>> + op_status_offset = xdr_stream_pos(xdr);
>> + if (!xdr_reserve_space(xdr, 4))
>
> The underlying message of this bug seems to be that xdr_reserve_space()
> is a low-level interface that probably shouldn't be used outside of xdr
> code.
> So I wonder if we could use
> op_status_offset = xdr_stream_pos(xdr);
> if (xdr_stream_encode_u32(xdr, NFS4ERR_SERVERFAULT) < 0) //will be over-written
> goto release;
> post_status_offset = xdr_stream_pos(xdr);
>
> instead??
>
> But these are minor thoughts - only use them if you like them.
> Generally this is a definite improvement.
>
> Reviewed-by: NeilBrown <neilb@suse.de>

Thanks!

--
Chuck Lever