tree 67ea3fc709cdfb79953f9d189be9a81d3a87a3f5
parent 41b2e6eff1da435174fe9c01b23b50f60b718ea1
author Chirantan Ekbote <chirantan@chromium.org> 1531787729 -0700
committer Greg Kroah-Hartman <gregkh@linuxfoundation.org> 1536516079 +0200

9p/net: Fix zero-copy path in the 9p virtio transport

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@chromium.org
Signed-off-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Cc: Dylan Reid <dgreid@chromium.org>
Cc: Guenter Roeck <groeck@chromium.org>
Cc: stable@vger.kernel.org
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>