Merge branch 'bpf-kernel-functions-with-kf_implicit_args'

Merge branch 'bpf-kernel-functions-with-kf_implicit_args'

Ihor Solodrai says:

====================
bpf: Kernel functions with KF_IMPLICIT_ARGS

This series implements a generic "implicit arguments" feature for BPF
kernel functions. For context see prior work [1][2].

A mechanism is created for kfuncs to have arguments that are not
visible to the BPF programs, and are provided to the kernel function
implementation by the verifier.

This mechanism is then used in the kfuncs that have a parameter with
__prog annotation [3], which is the current way of passing struct
bpf_prog_aux pointer to kfuncs.

The function with implicit arguments is defined by KF_IMPLICIT_ARGS
flag in BTF_IDS_FLAGS set. In this series, only a pointer to struct
bpf_prog_aux can be implicit, although it is simple to extend this to
more types.

The verifier handles a kfunc with KF_IMPLICIT_ARGS by resolving it to
a different (actual) BTF prototype early in verification (patch #3).

A <kfunc>_impl function generated in BTF for a kfunc with implicit
args does not have a "bpf_kfunc" decl tag, and a kernel address. The
verifier will reject a program trying to call such an _impl kfunc.

The usage of <kfunc>_impl functions in BPF is only allowed for kfuncs
with an explicit kernel (or kmodule) declaration, that is in "legacy"
cases. As of this series, there are no legacy kernel functions, as all
__prog users are migrated to KF_IMPLICIT_ARGS. However the
implementation allows for legacy cases support in principle.

The series removes the following BPF kernel functions:
    - bpf_stream_vprintk_impl
    - bpf_task_work_schedule_resume_impl
    - bpf_task_work_schedule_signal_impl
    - bpf_wq_set_callback_impl

This will break existing BPF programs calling these functions (the
verifier will not load them) on new kernels.

To mitigate, BPF users are advised to use the following pattern [4]:

    if (xxx_impl)
        xxx_impl(..., NULL);
    else
        xxx(...);

Which can be wrapped in a macro.

The series consists of the following patches:
  - patches #1 and #2 are non-functional refactoring in kernel/bpf
  - patch #3 defines KF_IMPLICIT_ARGS flag and teaches the verifier
    about it
  - patches #4-#5 implement btf2btf transformation in resolve_btfids
  - patch #6 adds selftests specific to KF_IMPLICIT_ARGS feature
  - patches #7-#11 migrate the current users of __prog argument to
    KF_IMPLICIT_ARGS
  - patch #12 removes __prog arg suffix support from the kernel
  - patch #13 updates the docs

[1] https://lore.kernel.org/bpf/20251029190113.3323406-1-ihor.solodrai@linux.dev/
[2] https://lore.kernel.org/bpf/20250924211716.1287715-1-ihor.solodrai@linux.dev/
[3] https://docs.kernel.org/bpf/kfuncs.html#prog-annotation
[4] https://lore.kernel.org/bpf/CAEf4BzbgPfRm9BX=TsZm-TsHFAHcwhPY4vTt=9OT-uhWqf8tqw@mail.gmail.com/
---

v2->v3:
  - resolve_btfids: Use dynamic reallocation for btf2btf_context arrays (Andrii)
  - resolve_btfids: Add missing free() for btf2btf_context arrays (AI)
  - Other nits in resolve_btfids (Andrii, Eduard)

v2: https://lore.kernel.org/bpf/20260116201700.864797-1-ihor.solodrai@linux.dev/

v1->v2:
  - Replace the following kernel functions with KF_IMPLICIT_ARGS version:
    - bpf_stream_vprintk_impl -> bpf_stream_vprintk
    - bpf_task_work_schedule_resume_impl -> bpf_task_work_schedule_resume
    - bpf_task_work_schedule_signal_impl -> bpf_task_work_schedule_signal
    - bpf_wq_set_callback_impl -> bpf_wq_set_callback_impl
  - Remove __prog arg suffix support from the verifier
  - Rework btf2btf implementation in resolve_btfids
    - Do distill base and sort before BTF_ids patching
    - Collect kfuncs based on BTF decl tags, before BTF_ids are patched
  - resolve_btfids: use dynamic memory for intermediate data (Andrii)
  - verifier: reset .subreg_def for caller saved registers on kfunc
    call (Eduard)
  - selftests/hid: remove Makefile changes (Benjamin)
  - selftests/bpf: Add a patch (#11) migrating struct_ops_assoc test
    to KF_IMPLICIT_ARGS
  - Various nits across the series (Alexei, Andrii, Eduard)

v1: https://lore.kernel.org/bpf/20260109184852.1089786-1-ihor.solodrai@linux.dev/

---
====================

Link: https://patch.msgid.link/20260120222638.3976562-1-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>