btf_prepare_func_args() is used to understand expectations and
restrictions on global subprog arguments. But current implementation is
hard to extend, as it intermixes BTF-based func prototype parsing and
interpretation logic with setting up register state at subprog entry.

Worse still, those registers are not completely set up inside
btf_prepare_func_args(), requiring some more logic later in
do_check_common(). Like calling mark_reg_unknown() and similar
initialization operations.

This intermixing of BTF interpretation and register state setup is
problematic. First, it causes duplication of BTF parsing logic for global
subprog verification (to set up initial state of global subprog) and
global subprog call sites analysis (when we need to check that whatever
is being passed into global subprog matches expectations), performed in
btf_check_subprog_call().

Given we want to extend global func argument with tags later, this
duplication is problematic. So refactor btf_prepare_func_args() to do
only BTF-based func proto and args parsing, returning high-level
argument "expectations" only, with no regard to specifics of register
state. I.e., if it's a context argument, instead of setting register
state to PTR_TO_CTX, we return ARG_PTR_TO_CTX enum for that argument as
"an argument specification" for further processing inside
do_check_common(). Similarly for SCALAR arguments, PTR_TO_MEM, etc.

This allows to reuse btf_prepare_func_args() in following patches at
global subprog call site analysis time. It also keeps register setup
code consistently in one place, do_check_common().

Besides all this, we cache this argument specs information inside
env->subprog_info, eliminating the need to redo these potentially
expensive BTF traversals, especially if BPF program's BTF is big and/or
there are lots of global subprog calls.
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231215011334.2307144-2-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>