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Subject: Re: [PATCH 3/3] NFS: Avoid copy of xdr padding in read()
From:   Chuck Lever <chuck.lever@oracle.com>
In-Reply-To: <57b085d32f624986412770d10cc4daa8211ee0f4.camel@hammerspace.com>
Date:   Thu, 19 Nov 2020 09:31:56 -0500
Cc:     "trondmy@kernel.org" <trondmy@kernel.org>,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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Message-Id: <D322F599-E680-4715-AD9A-CC6017AFF8E0@oracle.com>
References: <20201118221939.20715-1-trondmy@kernel.org>
 <20201118221939.20715-2-trondmy@kernel.org>
 <20201118221939.20715-3-trondmy@kernel.org>
 <42FFB4EC-5E31-4002-92FC-7CA329479D78@oracle.com>
 <57b085d32f624986412770d10cc4daa8211ee0f4.camel@hammerspace.com>
To:     Trond Myklebust <trondmy@hammerspace.com>
Precedence: bulk


> On Nov 19, 2020, at 9:30 AM, Trond Myklebust <trondmy@hammerspace.com> =
wrote:
>=20
> On Thu, 2020-11-19 at 09:17 -0500, Chuck Lever wrote:
>>=20
>>=20
>>> On Nov 18, 2020, at 5:19 PM, trondmy@kernel.org wrote:
>>>=20
>>> From: Trond Myklebust <trond.myklebust@hammerspace.com>
>>>=20
>>> When doing a read() into a page, we also don't care if the nul
>>> padding
>>> stays in that last page when the data length is not 32-bit aligned.
>>=20
>> What if the READ payload lands in the middle of a file? The
>> pad on the end will overwrite file content just past where
>> the READ payload lands.
>=20
> If the size > buf->page_len, then it gets truncated in
> xdr_align_pages() afaik.

I will need to check how RPC/RDMA behaves. It might build a
chunk that includes the pad in this case, which would break
things.


>>> Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
>>> ---
>>> fs/nfs/nfs2xdr.c | 2 +-
>>> fs/nfs/nfs3xdr.c | 2 +-
>>> fs/nfs/nfs4xdr.c | 2 +-
>>> 3 files changed, 3 insertions(+), 3 deletions(-)
>>>=20
>>> diff --git a/fs/nfs/nfs2xdr.c b/fs/nfs/nfs2xdr.c
>>> index db9c265ad9e1..468bfbfe44d7 100644
>>> --- a/fs/nfs/nfs2xdr.c
>>> +++ b/fs/nfs/nfs2xdr.c
>>> @@ -102,7 +102,7 @@ static int decode_nfsdata(struct xdr_stream
>>> *xdr, struct nfs_pgio_res *result)
>>>         if (unlikely(!p))
>>>                 return -EIO;
>>>         count =3D be32_to_cpup(p);
>>> -       recvd =3D xdr_read_pages(xdr, count);
>>> +       recvd =3D xdr_read_pages(xdr, xdr_align_size(count));
>>>         if (unlikely(count > recvd))
>>>                 goto out_cheating;
>>> out:
>>> diff --git a/fs/nfs/nfs3xdr.c b/fs/nfs/nfs3xdr.c
>>> index d3e1726d538b..8ef7c961d3e2 100644
>>> --- a/fs/nfs/nfs3xdr.c
>>> +++ b/fs/nfs/nfs3xdr.c
>>> @@ -1611,7 +1611,7 @@ static int decode_read3resok(struct
>>> xdr_stream *xdr,
>>>         ocount =3D be32_to_cpup(p++);
>>>         if (unlikely(ocount !=3D count))
>>>                 goto out_mismatch;
>>> -       recvd =3D xdr_read_pages(xdr, count);
>>> +       recvd =3D xdr_read_pages(xdr, xdr_align_size(count));
>>>         if (unlikely(count > recvd))
>>>                 goto out_cheating;
>>> out:
>>> diff --git a/fs/nfs/nfs4xdr.c b/fs/nfs/nfs4xdr.c
>>> index 755b556e85c3..5baa767106dc 100644
>>> --- a/fs/nfs/nfs4xdr.c
>>> +++ b/fs/nfs/nfs4xdr.c
>>> @@ -5202,7 +5202,7 @@ static int decode_read(struct xdr_stream
>>> *xdr, struct rpc_rqst *req,
>>>                 return -EIO;
>>>         eof =3D be32_to_cpup(p++);
>>>         count =3D be32_to_cpup(p);
>>> -       recvd =3D xdr_read_pages(xdr, count);
>>> +       recvd =3D xdr_read_pages(xdr, xdr_align_size(count));
>>>         if (count > recvd) {
>>>                 dprintk("NFS: server cheating in read reply: "
>>>                                 "count %u > recvd %u\n", count,
>>> recvd);
>>> --=20
>>> 2.28.0
>>>=20
>>=20
>> --
>> Chuck Lever
>>=20
>>=20
>>=20
>=20
> --=20
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com

--
Chuck Lever