Commit d28c756c authored Jul 16, 2018 by Chirantan Ekbote Committed by Dominique Martinet Aug 13, 2018

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

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>

parent 2557d0c5

net/9p/trans_virtio.c

+7 −0

Original line number	Diff line number	Diff line
		@@ -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);

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