bpf: Fix a verifier failure with xor

bpf: Fix a verifier failure with xor

bpf selftest test_progs/test_sk_assign failed with llvm 11 and llvm 12.
Compared to llvm 10, llvm 11 and 12 generates xor instruction which
is not handled properly in verifier. The following illustrates the
problem:

  16: (b4) w5 = 0
  17: ... R5_w=inv0 ...
  ...
  132: (a4) w5 ^= 1
  133: ... R5_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) ...
  ...
  37: (bc) w8 = w5
  38: ... R5=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff))
          R8_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) ...
  ...
  41: (bc) w3 = w8
  42: ... R3_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) ...
  45: (56) if w3 != 0x0 goto pc+1
   ... R3_w=inv0 ...
  46: (b7) r1 = 34
  47: R1_w=inv34 R7=pkt(id=0,off=26,r=38,imm=0)
  47: (0f) r7 += r1
  48: R1_w=invP34 R3_w=inv0 R7_w=pkt(id=0,off=60,r=38,imm=0)
  48: (b4) w9 = 0
  49: R1_w=invP34 R3_w=inv0 R7_w=pkt(id=0,off=60,r=38,imm=0)
  49: (69) r1 = *(u16 *)(r7 +0)
  invalid access to packet, off=60 size=2, R7(id=0,off=60,r=38)
  R7 offset is outside of the packet

At above insn 132, w5 = 0, but after w5 ^= 1, we give a really conservative
value of w5. At insn 45, in reality the condition should be always false.
But due to conservative value for w3, the verifier evaluates it could be
true and this later leads to verifier failure complaining potential
packet out-of-bound access.

This patch implemented proper XOR support in verifier.
In the above example, we have:
  132: R5=invP0
  132: (a4) w5 ^= 1
  133: R5_w=invP1
  ...
  37: (bc) w8 = w5
  ...
  41: (bc) w3 = w8
  42: R3_w=invP1
  ...
  45: (56) if w3 != 0x0 goto pc+1
  47: R3_w=invP1
  ...
  processed 353 insns ...
and the verifier can verify the program successfully.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200825064608.2017937-1-yhs@fb.com