Re: [PATCH bpf v2 3/5] libbpf: Skip adjust mem size for load pointer in 32-bit arch in CO_RE

[Yang Jihong]

Hello,

On 2022/11/8 9:22, Andrii Nakryiko wrote:
> On Mon, Nov 7, 2022 at 1:23 AM Yang Jihong <yangjihong1@xxxxxxxxxx> wrote:
> > bpf_core_patch_insn modifies load's mem size from 8 bytes to 4 bytes.
> > As a result, the bpf check fails, we need to skip adjust mem size to fit
> > the verifier.
> >
> > Signed-off-by: Yang Jihong <yangjihong1@xxxxxxxxxx>
> > ---
> >   tools/lib/bpf/libbpf.c | 34 +++++++++++++++++++++++++++++-----
> >   1 file changed, 29 insertions(+), 5 deletions(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 184ce1684dcd..e1c21b631a0b 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -5634,6 +5634,28 @@ static int bpf_core_resolve_relo(struct bpf_program *prog,
> >                                         targ_res);
> >   }
> >
> > +static bool
> > +bpf_core_patch_insn_skip(const struct btf *local_btf, const struct bpf_insn *insn,
> > +                        const struct bpf_core_relo_res *res)
> > +{
> > +       __u8 class;
> > +       const struct btf_type *orig_t;
> > +
> > +       class = BPF_CLASS(insn->code);
> > +       orig_t = btf_type_by_id(local_btf, res->orig_type_id);
> > +
> > +       /*
> > +        * verifier has to see a load of a pointer as a 8-byte load,
> > +        * CO_RE should not screws up access, bpf_core_patch_insn modifies
> > +        * load's mem size from 8 bytes to 4 bytes in 32-bit arch,
> > +        * so we skip adjust mem size.
> > +        */
>
> Nope, this is only for BPF UAPI context types like __sk_buff (right
> now). fentry/fexit/raw_tp_btf programs traversing kernel types and
> following pointers actually need this to work correctly. Don't do
> this.
Distinguishing BPF UAPI context from kernel type requires some work. 
According to current situation, the solution of patch2 is relatively simple.

Thanks,
Yang