[External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support to record and check the accessed args

[Alexei Starovoitov]

On Mon, Mar 11, 2024 at 7:01 PM 梦龙董 <dongmenglong.8 at bytedance.com> wrote:
>
> On Tue, Mar 12, 2024 at 9:46 AM Alexei Starovoitov
> <alexei.starovoitov at gmail.com> wrote:
> >
> > On Mon, Mar 11, 2024 at 2:34 AM Menglong Dong
> > <dongmenglong.8 at bytedance.com> wrote:
> > >
> > > In this commit, we add the 'accessed_args' field to struct bpf_prog_aux,
> > > which is used to record the accessed index of the function args in
> > > btf_ctx_access().
> > >
> > > Meanwhile, we add the function btf_check_func_part_match() to compare the
> > > accessed function args of two function prototype. This function will be
> > > used in the following commit.
> > >
> > > Signed-off-by: Menglong Dong <dongmenglong.8 at bytedance.com>
> > > ---
> > >  include/linux/bpf.h |   4 ++
> > >  kernel/bpf/btf.c    | 108 +++++++++++++++++++++++++++++++++++++++++++-
> > >  2 files changed, 110 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > > index 95e07673cdc1..0f677fdcfcc7 100644
> > > --- a/include/linux/bpf.h
> > > +++ b/include/linux/bpf.h
> > > @@ -1461,6 +1461,7 @@ struct bpf_prog_aux {
> > >         const struct btf_type *attach_func_proto;
> > >         /* function name for valid attach_btf_id */
> > >         const char *attach_func_name;
> > > +       u64 accessed_args;
> > >         struct bpf_prog **func;
> > >         void *jit_data; /* JIT specific data. arch dependent */
> > >         struct bpf_jit_poke_descriptor *poke_tab;
> > > @@ -2565,6 +2566,9 @@ struct bpf_reg_state;
> > >  int btf_prepare_func_args(struct bpf_verifier_env *env, int subprog);
> > >  int btf_check_type_match(struct bpf_verifier_log *log, const struct bpf_prog *prog,
> > >                          struct btf *btf, const struct btf_type *t);
> > > +int btf_check_func_part_match(struct btf *btf1, const struct btf_type *t1,
> > > +                             struct btf *btf2, const struct btf_type *t2,
> > > +                             u64 func_args);
> > >  const char *btf_find_decl_tag_value(const struct btf *btf, const struct btf_type *pt,
> > >                                     int comp_idx, const char *tag_key);
> > >  int btf_find_next_decl_tag(const struct btf *btf, const struct btf_type *pt,
> > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > index 170d017e8e4a..c2a0299d4358 100644
> > > --- a/kernel/bpf/btf.c
> > > +++ b/kernel/bpf/btf.c
> > > @@ -6125,19 +6125,24 @@ static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
> > >  }
> > >
> > >  static u32 get_ctx_arg_idx(struct btf *btf, const struct btf_type *func_proto,
> > > -                          int off)
> > > +                          int off, int *aligned_idx)
> > >  {
> > >         const struct btf_param *args;
> > >         const struct btf_type *t;
> > >         u32 offset = 0, nr_args;
> > >         int i;
> > >
> > > +       if (aligned_idx)
> > > +               *aligned_idx = -ENOENT;
> > > +
> > >         if (!func_proto)
> > >                 return off / 8;
> > >
> > >         nr_args = btf_type_vlen(func_proto);
> > >         args = (const struct btf_param *)(func_proto + 1);
> > >         for (i = 0; i < nr_args; i++) {
> > > +               if (aligned_idx && offset == off)
> > > +                       *aligned_idx = i;
> > >                 t = btf_type_skip_modifiers(btf, args[i].type, NULL);
> > >                 offset += btf_type_is_ptr(t) ? 8 : roundup(t->size, 8);
> > >                 if (off < offset)
> > > @@ -6207,7 +6212,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> > >                         tname, off);
> > >                 return false;
> > >         }
> > > -       arg = get_ctx_arg_idx(btf, t, off);
> > > +       arg = get_ctx_arg_idx(btf, t, off, NULL);
> > >         args = (const struct btf_param *)(t + 1);
> > >         /* if (t == NULL) Fall back to default BPF prog with
> > >          * MAX_BPF_FUNC_REG_ARGS u64 arguments.
> > > @@ -6217,6 +6222,9 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> > >                 /* skip first 'void *__data' argument in btf_trace_##name typedef */
> > >                 args++;
> > >                 nr_args--;
> > > +               prog->aux->accessed_args |= (1 << (arg + 1));
> > > +       } else {
> > > +               prog->aux->accessed_args |= (1 << arg);
> >
> > What do you need this aligned_idx for ?
> > I'd expect that above "accessed_args |= (1 << arg);" is enough.
> >
>
> Which aligned_idx? No aligned_idx in the btf_ctx_access(), and
> aligned_idx is only used in the btf_check_func_part_match().
>
> In the btf_check_func_part_match(), I need to compare the
> t1->args[i] and t2->args[j], which have the same offset. And
> the aligned_idx is to find the "j" according to the offset of
> t1->args[i].

And that's my question.
Why you don't do the max of accessed_args across all attach
points and do btf_check_func_type_match() to that argno
instead of nargs1.
This 'offset += btf_type_is_ptr(t1) ? 8 : roundup...
is odd.