Merge branch 'bpf-enforce-trusted-btf-id-argument-provenance'

Yiyang Chen says:

====================
bpf: Enforce trusted BTF ID argument provenance

A global subprogram parameter tagged __arg_trusted is verified with
PTR_TRUSTED, but its call-site type check also accepts bare PTR_TO_BTF_ID and
MEM_RCU registers. This lets a callee use a pointer with weaker provenance
under the trusted-pointer contract, including passing it to a non-KF_RCU
kfunc.

Encode the trusted-pointer requirement in generated kfunc prototypes and
drive the shared provenance check from the argument contract. A KF_RCU
kfunc additionally permits MEM_RCU, while a global __arg_trusted argument
requires a referenced or trusted pointer. Normalize nullability when the
argument contract permits it.

Add rejection coverage for bare and RCU-protected global-subprogram arguments
and acceptance coverage for a nullable kfunc receiving a trusted nullable
pointer.

A separate scx_cake caller fix has been prepared but is not yet submitted.
The current veristat-scx pin still uses the old caller, which this series
rejects.

Changes in v4:
- Rebase onto bpf-next and use the shared argument checker.
- Reject MEM_RCU for global __arg_trusted arguments.
- Encode trusted and KF_RCU provenance in generated kfunc prototypes.
- Add nullable-kfunc coverage and use a 12-character Fixes SHA.
- Link to v3: https://lore.kernel.org/r/20261006-a3-arg-trusted-v4-v3-0-4619daf30e1c@mails.tsinghua.edu.cn

Changes in v3:
- Preserve RCU-protected arguments accepted by existing sched-ext programs.
- Make the MEM_RCU case a positive regression test.
- Use the preferred multi-line comment style.
- Link to v2: https://lore.kernel.org/r/20261005-a3-arg-trusted-v4-v2-0-319ce2936949@mails.tsinghua.edu.cn

Changes in v2:
- Retarget the fix to btf_check_func_arg_match(), where the subprogram
  argument check lives in this tree, instead of check_func_arg().
- Run the check after check_reg_type() and check_func_arg_reg_off() so type
  and offset diagnostics keep their wording.

v1: https://lore.kernel.org/bpf/20261005-a3-arg-trusted-v4-v1-0-50ee0268fd39@mails.tsinghua.edu.cn/
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====================

Link: https://patch.msgid.link/20261007-a3-arg-trusted-v4-v4-0-253ef3a1a69a@mails.tsinghua.edu.cn
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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