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

David Vernet
2023-01-07T05:27:22Z

On Fri, Jan 06, 2023 at 04:47:35PM -0800, Stanislav Fomichev wrote:
> 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:

Thanks for the review, Stanislav. I agree that it would be nice to have
some automation to prevent forgetting the tag. I thought about ways to
possibly do it, including playing around with putting the kfuncs into a
separate section for post-processing which we could check against
.BTF_ids, but it felt like a lot of complexity / possibly controversial
changes that I'm hesitant to bring into the patch set which should be
pretty non-controversial otherwise.

With respect to validating the presence of kfunc "tags" (i.e. the
__diag_push() / __diag_pop() we were doing before), we're in the same
state after this patch as we were before, so my preference is to defer
improving that until a later time when we've fried some of the bigger
kfunc fish. Does that sound ok?

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

Thanks! FYI, I'm planning on sending a v2 with Alexei's suggestion [0]

[0]: https://lore.kernel.org/all/CAADnVQLpK7WXTjF6GS1hcfPXf=8iERJmEeVFfvmG75mJj0DdaA@mail.gmail.com/

I'll go ahead and preemptively leave off your Acked-by for that, as the
patches will have changed enough that it probably warrants another
read through.

- David

>
>
>
>
> > 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
> >