Currently, when considering the branches that may be taken for a jump
instruction if the register being compared is a pointer the verifier
assumes both branches may be taken. But, if the jump instruction
is comparing if a pointer is NULL we have this information in the
verifier encoded in the reg->type so we can do better in these cases.
Specifically, these two common cases can be handled.

 * If the instruction is BPF_JEQ and we are comparing against a
   zero value. This test is 'if ptr == 0 goto +X' then using the
   type information in reg->type we can decide if the ptr is not
   null. This allows us to avoid pushing both branches onto the
   stack and instead only use the != 0 case. For example
   PTR_TO_SOCK and PTR_TO_SOCK_OR_NULL encode the null pointer.
   Note if the type is PTR_TO_SOCK_OR_NULL we can not learn anything.
   And also if the value is non-zero we learn nothing because it
   could be any arbitrary value a different pointer for example

 * If the instruction is BPF_JNE and ware comparing against a zero
   value then a similar analysis as above can be done. The test in
   asm looks like 'if ptr != 0 goto +X'. Again using the type
   information if the non null type is set (from above PTR_TO_SOCK)
   we know the jump is taken.

In this patch we extend is_branch_taken() to consider this extra
information and to return only the branch that will be taken. This
resolves a verifier issue reported with C code like the following.
See progs/test_sk_lookup_kern.c in selftests.

 sk = bpf_sk_lookup_tcp(skb, tuple, tuple_len, BPF_F_CURRENT_NETNS, 0);
 bpf_printk("sk=%d\n", sk ? 1 : 0);
 if (sk)
   bpf_sk_release(sk);
 return sk ? TC_ACT_OK : TC_ACT_UNSPEC;

In the above the bpf_printk() will resolve the pointer from
PTR_TO_SOCK_OR_NULL to PTR_TO_SOCK. Then the second test guarding
the release will cause the verifier to walk both paths resulting
in the an unreleased sock reference. See verifier/ref_tracking.c
in selftests for an assembly version of the above.

After the above additional logic is added the C code above passes
as expected.
Reported-by: Andrey Ignatov <rdna@fb.com>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/159009164651.6313.380418298578070501.stgit@john-Precision-5820-Tower