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

NeilBrown
2024-12-24T23:19:11Z

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;

> 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,
NeilBrown