[PATCH] net: filter: make JITs zero A for SKF_AD_ALU_XOR_X

On Tue, 2016-01-05 at 16:23 +0100, Rabin Vincent wrote:

The SKF_AD_ALU_XOR_X ancillary is not like the other ancillary data
instructions since it XORs A with X while all the others replace A with
some loaded value.  All the BPF JITs fail to clear A if this is used as
the first instruction in a filter.

Is x86_64 part of this 'All' subset ? ;)

  This was found using american fuzzy
lop.

Add a helper to determine if A needs to be cleared given the first
instruction in a filter, and use this in the JITs.  Except for ARM, the
rest have only been compile-tested.

Fixes: 3480593131e0 ("net: filter: get rid of BPF_S_* enum")
Signed-off-by: Rabin Vincent <redacted>
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