bpf: Handle BPF_UPTR in verifier

This patch adds BPF_UPTR support to the verifier. Not that only the
map_value will support the "__uptr" type tag.

This patch enforces only BPF_LDX is allowed to the value of an uptr.
After BPF_LDX, it will mark the dst_reg as PTR_TO_MEM | PTR_MAYBE_NULL
with size deduced from the field.kptr.btf_id. This will make the
dst_reg pointed memory to be readable and writable as scalar.

There is a redundant "val_reg = reg_state(env, value_regno);" statement
in the check_map_kptr_access(). This patch takes this chance to remove
it also.

Signed-off-by: Kui-Feng Lee <[email protected]>
Signed-off-by: Martin KaFai Lau <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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