Chirantan Ekbote wrote on Mon, Jul 16, 2018:
> The zero-copy optimization when reading or writing large chunks of data
> is quite useful. However, the 9p messages created through the zero-copy
> write path have an incorrect message size: it should be the size of the
> header + size of the data being written but instead it's just the size
> of the header.
>
> This only works if the server ignores the size field of the message and
> otherwise breaks the framing of the protocol. Fix this by re-writing the
> message size field with the correct value.
>
> Tested by running `dd if=/dev/zero of=out bs=4k count=1` inside a
> virtio-9p mount.
>
> Signed-off-by: Chirantan Ekbote <chiran...@chromium.org>
> Reviewed-by: Greg Kurz <gr...@kaod.org>
> Tested-by: Greg Kurz <gr...@kaod.org>
Ack, I've added this to my queue for 4.19
Thanks for moving the memcpy and the updated comment, it makes it more
clear that these are different fields of the message.
> ---
> net/9p/trans_virtio.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/net/9p/trans_virtio.c b/net/9p/trans_virtio.c
> index 05006cbb3361..65761381c58f 100644
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -406,6 +406,7 @@ p9_virtio_zc_request(struct p9_client *client, struct
> p9_req_t *req,
> p9_debug(P9_DEBUG_TRANS, "virtio request\n");
>
> if (uodata) {
> + __le32 sz;
> int n = p9_get_mapped_pages(chan, &out_pages, uodata,
> outlen, &offs, &need_drop);
> if (n < 0)
> @@ -416,6 +417,12 @@ p9_virtio_zc_request(struct p9_client *client, struct
> p9_req_t *req,
> memcpy(&req->tc->sdata[req->tc->size - 4], &v, 4);
> outlen = n;
> }
> + /* The size field of the message must include the length of the
> + * header and the length of the data. We didn't actually know
> + * the length of the data until this point so add it in now.
> + */
> + sz = cpu_to_le32(req->tc->size + outlen);
> + memcpy(&req->tc->sdata[0], &sz, sizeof(sz));
> } else if (uidata) {
> int n = p9_get_mapped_pages(chan, &in_pages, uidata,
> inlen, &offs, &need_drop);
--
Dominique Martinet
