Re: [PATCH v2 bpf-next 5/7] bpf: support BPF ksym variables in kernel modules

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Mon, Jan 11, 2021 at 11:00 AM Hao Luo <haoluo@google.com> wrote:
>
> Acked-by: Hao Luo <haoluo@google.com>, with a suggestion on adding a comment.
>

top posting your Ack? :)


> On Fri, Jan 8, 2021 at 2:09 PM Andrii Nakryiko <andrii@kernel.org> wrote:
> >
> > Add support for directly accessing kernel module variables from BPF programs
> > using special ldimm64 instructions. This functionality builds upon vmlinux
> > ksym support, but extends ldimm64 with src_reg=BPF_PSEUDO_BTF_ID to allow
> > specifying kernel module BTF's FD in insn[1].imm field.
> >
> > During BPF program load time, verifier will resolve FD to BTF object and will
> > take reference on BTF object itself and, for module BTFs, corresponding module
> > as well, to make sure it won't be unloaded from under running BPF program. The
> > mechanism used is similar to how bpf_prog keeps track of used bpf_maps.
> >
> > One interesting change is also in how per-CPU variable is determined. The
> > logic is to find .data..percpu data section in provided BTF, but both vmlinux
> > and module each have their own .data..percpu entries in BTF. So for module's
> > case, the search for DATASEC record needs to look at only module's added BTF
> > types. This is implemented with custom search function.
> >
> > Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> > ---
> >  include/linux/bpf.h          |  10 +++
> >  include/linux/bpf_verifier.h |   3 +
> >  include/linux/btf.h          |   3 +
> >  kernel/bpf/btf.c             |  31 +++++++-
> >  kernel/bpf/core.c            |  23 ++++++
> >  kernel/bpf/verifier.c        | 149 ++++++++++++++++++++++++++++-------
> >  6 files changed, 189 insertions(+), 30 deletions(-)
>
> [...]
>
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 17270b8404f1..af94c6871ab8 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -9703,6 +9703,31 @@ static int do_check(struct bpf_verifier_env *env)
> >         return 0;
> >  }
> >
> > +static int find_btf_percpu_datasec(struct btf *btf)
> > +{
> > +       const struct btf_type *t;
> > +       const char *tname;
> > +       int i, n;
> > +
>
> It would be good to add a short comment here explaining the reason why
> the search for DATASEC in the module case needs to skip entries.

I can copy-paste parts of the commit message with that explanation, if
I'll need another version. If not, I can send a follow-up patch.

>
> > +       n = btf_nr_types(btf);
> > +       if (btf_is_module(btf))
> > +               i = btf_nr_types(btf_vmlinux);
> > +       else
> > +               i = 1;
> > +
> > +       for(; i < n; i++) {
> > +               t = btf_type_by_id(btf, i);
> > +               if (BTF_INFO_KIND(t->info) != BTF_KIND_DATASEC)
> > +                       continue;
> > +
> > +               tname = btf_name_by_offset(btf, t->name_off);
> > +               if (!strcmp(tname, ".data..percpu"))
> > +                       return i;
> > +       }
> > +
> > +       return -ENOENT;
> > +}
> [...]
> > 2.24.1
> >