[PATCH bpf-next 0/3] Annotate kfuncs with new __bpf_kfunc macro

[David Vernet <void@manifault.com>]

BPF kfuncs are kernel functions that can be invoked by BPF programs.
kfuncs can be kernel functions which are also called elsewhere in the
main kernel (such as crash_kexec()), or may be functions that are only
meant to be used by BPF programs, such as bpf_task_acquire(), and which
are not called from anywhere else in the kernel.

While thus far we haven't observed any issues such as kfuncs being
elided by the compiler, at some point we could easily run into problems
such as the following:

- static kernel functions that are also used as kfuncs could be inlined
  and/or elided by the compiler.
- BPF-specific kfuncs with external linkage may at some point be elided
  by the compiler in LTO builds, when it's determined that they aren't
  called anywhere.

To address this, this patch set introduces a new __bpf_kfunc macro which
should be added to all kfuncs, and which will protect kfuncs from such
problems. Note that some kfuncs kind of try to do this already by
specifying noinline or __used. We are inconsistent in how this is
applied. __bpf_kfunc should provide a uniform and more-future-proof way
to do this.

David Vernet (3):
  bpf: Add __bpf_kfunc tag for marking kernel functions as kfuncs
  bpf: Document usage of the new __bpf_kfunc macro
  bpf: Add __bpf_kfunc tag to all kfuncs

 Documentation/bpf/kfuncs.rst                  | 18 +++++
 Documentation/conf.py                         |  3 +
 include/linux/btf.h                           |  9 +++
 kernel/bpf/helpers.c                          | 19 +++++
 kernel/cgroup/rstat.c                         |  2 +
 kernel/kexec_core.c                           |  2 +
 kernel/trace/bpf_trace.c                      |  4 +
 net/bpf/test_run.c                            | 76 ++++++++++++-------
 net/ipv4/tcp_bbr.c                            |  8 ++
 net/ipv4/tcp_cong.c                           |  5 ++
 net/ipv4/tcp_cubic.c                          |  6 ++
 net/ipv4/tcp_dctcp.c                          |  6 ++
 net/netfilter/nf_conntrack_bpf.c              | 14 +++-
 net/netfilter/nf_nat_bpf.c                    |  1 +
 net/xfrm/xfrm_interface_bpf.c                 |  4 +-
 .../selftests/bpf/bpf_testmod/bpf_testmod.c   |  2 +-
 16 files changed, 146 insertions(+), 33 deletions(-)

-- 
2.39.0