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

Stanislav Fomichev
2023-01-07T00:48:30Z

On Fri, Jan 6, 2023 at 11:51 AM David Vernet <void@manifault.com> wrote:
>
> 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.

The series looks reasonable to me. Would be nice if we can somehow
prevent (with a checkpatch?) adding new kfuncs without this new tag,
but I don't see an easy way.
I was waiting in case other would like to comment, but if nothing to discuss:

Acked-by: Stanislav Fomichev <sdf@google.com>

> 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
>