rxrpc: Fix alteration of headers whilst zerocopy pending
AF_RXRPC now uses MSG_SPLICE_PAGES to do zerocopy of the DATA packets when
it transmits them, but to reduce the number of descriptors required in the
DMA ring, it allocates a space for the protocol header in the memory
immediately before the data content so that it can include both in a single
descriptor. This is used for either the main RX header or the smaller
jumbo subpacket header as appropriate:
+----+------+
| RX | |
+-+--+DATA |
|JH| |
+--+------+
Now, when it stitches a large jumbo packet together from a number of
individual DATA packets (each of which is 1412 bytes of data), it uses the
full RX header from the first and then the jumbo subpacket header for the
rest of the components:
+---+--+------+--+------+--+------+--+------+--+------+--+------+
|UDP|RX|DATA |JH|DATA |JH|DATA |JH|DATA |JH|DATA |JH|DATA |
+---+--+------+--+------+--+------+--+------+--+------+--+------+
As mentioned, the main RX header and the jumbo header overlay one another
in memory and the formats don't match, so switching from one to the other
means rearranging the fields and adjusting the flags.
However, now that TLP has been included, it wants to retransmit the last
subpacket as a new data packet on its own, which means switching between
the header formats... and if the transmission is still pending, because of
the MSG_SPLICE_PAGES, we end up corrupting the jumbo subheader.
This has a variety of effects, with the RX service number overwriting the
jumbo checksum/key number field and the RX checksum overwriting the jumbo
flags - resulting in, at the very least, a confused connection-level abort
from the peer.
Fix this by leaving the jumbo header in the allocation with the data, but
allocating the RX header from the page frag allocator and concocting it on
the fly at the point of transmission as it does for ACK packets.
Fixes: 7c48266593 ("rxrpc: Implement RACK/TLP to deal with transmission stalls [RFC8985]")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/2181712.1739131675@warthog.procyon.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
06ea2c9c41