On 2020-07-19 19:34, Rick Macklem wrote:
I spent a little time chasing a problem in the nfs-over-tls code, where it
would sometimes end up with corrupted data in the file(s) of a mirrored
pNFS configuration.
I think the problem was that the code filled the data to be written into
anonymous page M_EXTPG mbufs, then did a m_copym() { copy by
reference } and used the copies for the mirrored writes.
--> In ktls_encrypt(), the encryption was done to the same pages and,
sometimes, the encrypted data got encrypted again during the
sosend() of the other copy.
Although I haven't reproduced it, a regular kernel write RPC could suffer the
same consequences if the RPC is retried (it keeps an m_copym() copy
of the request in the krpc for an RPC retry).
At this time, the code in projects/nfs-over-tls works correctly, since it
always fills the data to be written into mbuf clusters, m_copym()s those
and then copies those { real copying using memcpy() } via
mb_mapped_to_unmapped() just before calling sosend().
--> This works, but it would be nice to avoid the mb_mapped_to_unmapped()
copying for all the data being written via an NFS over TLS connection.
For the TCP_TLS_MODE_SW case:
--> The NFS code can fill the written data into anonymous pages on M_EXTPG
mbufs.
Then, the ktls_encrypt() could be modified to
allocate a new set of anonymous pages for the destination side of
the encryption (it already does this for the sendfile case) and put those
in a new mbuf list.
--> This would result in new anonymous pages and mbufs being allocated,
but would not do memcpy()s.
After encryption, it would just do a m_freem() on the unencrypted list.
--> For the krpc client case, this call would only decrement the reference
count on the unencrypted list and it could be used for a retry by the
krpc
and then be free'd { m_freem() call } after a reply is received.
If doing this for all the sosend()s of anonymous page M_EXTPG mbufs seems
like unnecessary overhead, the above could be enabled via a setsockopt()
on the socket.
What do others think of this?
Several comments:
mb_mapped_to_unmapped() is surprisingly inexpensive. It was less than
5% before I converted iflib to M_NOMAP aware.
It seems like NFS should be constructing mbufs like sendfile does, and
pointing mbufs at its pages. This would cause the crypto code to
allocate a new set of pages upon encryption.
For the hardware offload case:
- Can I assume that the anonymous pages in M_EXTPG mbufs will remain
unchanged?
--> If so, and it won't change to TCP_TLS_MODE_SW, the NFS code could
fill the data to be written into M_EXTPG mbufs safely.
- And, if so, can I safely use the ktls_session mode field to decide if offload
is happening?
I see the TCP_TXTLS_MODE socket opt which seems to
switch the mode to TCP_TLS_MODE_SW.
When does this happen? Or, can this happen to a session once in use?
Yes. The intent is to allow something (TCP stack, smart user daemon) to
look at a connection & move it from hardware to software, if it has a
lot of TCP re-transmits.
Drew
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