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 2:54 PM Guenter Roeck <groeck@xxxxxxxxxx> wrote:
>
> 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.
>
Hmm, I suspect it is the gmail UI messing with me. Sorry for the noise.

Guenter

> 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);
> >
> >