Alexei Starovoitov 735b433397 bpf: improve verifier state equivalence
since UNKNOWN_VALUE type is weaker than CONST_IMM we can un-teach
verifier its recognition of constants in conditional branches
without affecting safety.
Ex:
if (reg == 123) {
  .. here verifier was marking reg->type as CONST_IMM
     instead keep reg as UNKNOWN_VALUE
}

Two verifier states with UNKNOWN_VALUE are equivalent, whereas
CONST_IMM_X != CONST_IMM_Y, since CONST_IMM is used for stack range
verification and other cases.
So help search pruning by marking registers as UNKNOWN_VALUE
where possible instead of CONST_IMM.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
2016-05-06 16:01:54 -04:00
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