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

[David Vernet]

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@xxxxxxxxxxxxx> 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@xxxxxxxxxx>

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

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

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
> >