Re: [PATCH v4 9/9] SUNRPC: Document validity guarantees of the pointer returned by reserve_space

[Jeff Layton <jlayton@kernel.org>]

On Mon, 2024-12-30 at 19:29 -0500, cel@kernel.org wrote:
> From: Chuck Lever <chuck.lever@oracle.com>
> 
> A subtlety of this API is that if the @nbytes region traverses a
> page boundary, the next __xdr_commit_encode will shift the data item
> in the XDR encode buffer. This makes the returned pointer point to
> something else, leading to unexpected behavior.
> 
> There are a few cases where the caller saves the returned pointer
> and then later uses it to insert a computed value into an earlier
> part of the stream. This can be safe only if either:
> 
>  - the data item is guaranteed to be in the XDR buffer's head, and
>    thus is not ever going to be near a page boundary, or
>  - the data item is no larger than 4 octets, since XDR alignment
>    rules require all data items to start on 4-octet boundaries
> 
> But that safety is only an artifact of the current implementation.
> It would be less brittle if these "safe" uses were eventually
> replaced.
> 
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---
>  net/sunrpc/xdr.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/net/sunrpc/xdr.c b/net/sunrpc/xdr.c
> index 62e07c330a66..4e003cb516fe 100644
> --- a/net/sunrpc/xdr.c
> +++ b/net/sunrpc/xdr.c
> @@ -1097,6 +1097,12 @@ static noinline __be32 *xdr_get_next_encode_buffer(struct xdr_stream *xdr,
>   * Checks that we have enough buffer space to encode 'nbytes' more
>   * bytes of data. If so, update the total xdr_buf length, and
>   * adjust the length of the current kvec.
> + *
> + * The returned pointer is valid only until the next call to
> + * xdr_reserve_space() or xdr_commit_encode() on @xdr. The current
> + * implementation of this API guarantees that space reserved for a
> + * four-byte data item remains valid until @xdr is destroyed, but
> + * that might not always be true in the future.
>   */
>  __be32 * xdr_reserve_space(struct xdr_stream *xdr, size_t nbytes)
>  {

I agree with Neil that this API could do with less footguns, but this
does seem to be an improvement.

Reviewed-by: Jeff Layton <jlayton@kernel.org>