bpf: Admit kfunc argument registers through check_reg_type()

check_kfunc_args() open-codes exact register-type tests in most of
its per-argument cases, duplicating what compatible_reg_types[]
already expresses for helpers. This leaves two admission paths and
prevents the helper and kfunc loops from converging.

Runtime argument resolution now converts a scalar-struct BTF
argument to fixed-size memory before register admission. Give the
remaining kfunc-only argument kinds compatibility entries and run
check_reg_type() once before the per-kind switch.

Kfunc memory arguments already accept BPF-allocated objects.
Normalize only the local comparison type to PTR_TO_MEM; subsequent
memory checks still inspect the original register type. Keep
allocated-object forms out of mem_types so helper calls continue
through the existing type-mismatch path and retain its diagnostic.

For ARG_PTR_TO_BTF_ID, let check_reg_type() admit BTF-backed
register types and reject incompatible register classes with its
standard diagnostic. Remove the now-unused lookup_reg2btf_ids().
Exact BTF identity and trust requirements remain checked later by
process_arg_ptr_to_btf_id(). ARG_IGNORE and ARG_PTR_TO_PROG_AUX remain
skipped because the verifier does not read those arguments from the
program.

Iterator arguments use the stack-pointer table. Graph nodes and
ARG_PTR_TO_REFCOUNTED_KPTR share an allocated-object table. It
admits owning and borrowed objects, including RCU-protected forms.
Their switch cases retain API-specific ownership and BTF-record
validation.
ARG_PTR_TO_ALLOC_BTF_ID uses a separate table for object-drop arguments.

Rename timer_types to map_value_types now that ARG_PTR_TO_WORKQUEUE
and ARG_PTR_TO_TASK_WORK share it. Similarly, rename spin_lock_types
to map_value_or_alloc_obj_types because graph roots and resource spin
locks share its map-value-or-allocated-object admission.

Moving admission checks into check_reg_type() must not discard the
structured call-argument diagnostics emitted by the individual cases.
Add bpf_diag_arg_type_plain() alongside bpf_diag_reg_type_plain() and
use it to preserve the existing per-kind Pass suggestions where
available. Other argument kinds retain the generic suggestion. The
reason continues to report the actual register type and all accepted
register types.

Two behavior changes fall out of running admission first:

  - ARG_CONST_MEM_SIZE reaches process_const_arg(), and through it
    mark_chain_precision(), only after the register is known to be a
    scalar. Passing a pointer as a __szk argument used to reach
    backtrack_insn() with a non-scalar and trip the backtracking-misuse
    verifier bug.

  - ARG_CONST_MAP_PTR no longer needs its own type_may_be_null()
    test, because check_reg_type() compares whole register types.

Every kfunc argument that is not explicitly ignored now passes
through check_reg_type(), followed by the common register-offset check
in the same order as a helper argument.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260911220415.1396439-19-ameryhung@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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