Re: [PATCH 4.9 03/63] 9p/net: Fix zero-copy path in the 9p virtio transport

[Guenter Roeck]

On Fri, Sep 7, 2018 at 2:34 PM Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>
> 4.9-stable review patch.  If anyone has any objections, please let me know.
>

Do your scripts have a bad hair day ? The subject says 4.18.

Guenter

> ------------------
>
> From: Chirantan Ekbote <chirantan@xxxxxxxxxxxx>
>
> commit d28c756caee6e414d9ba367d0b92da24145af2a8 upstream.
>
> 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.
>
> Link: http://lkml.kernel.org/r/20180717003529.114368-1-chirantan@xxxxxxxxxxxx
> Signed-off-by: Chirantan Ekbote <chirantan@xxxxxxxxxxxx>
> Reviewed-by: Greg Kurz <groug@xxxxxxxx>
> Tested-by: Greg Kurz <groug@xxxxxxxx>
> Cc: Dylan Reid <dgreid@xxxxxxxxxxxx>
> Cc: Guenter Roeck <groeck@xxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Dominique Martinet <dominique.martinet@xxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>
> ---
>  net/9p/trans_virtio.c |    7 +++++++
>  1 file changed, 7 insertions(+)
>
> --- a/net/9p/trans_virtio.c
> +++ b/net/9p/trans_virtio.c
> @@ -406,6 +406,7 @@ p9_virtio_zc_request(struct p9_client *c
>         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 *c
>                         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);
>
>