linux-kernel - Re: [PATCH bpf-next v4 08/12] bpf: verifier: Relax caller requirements for kfunc projection type args

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Message-ID: <3ys25qg63cfuxjclqjlagsasvp5bpu6oqzjeia32kg2seistbv@5t24bsw5jtij>
Date: Tue, 11 Jun 2024 12:01:03 -0600
From: Daniel Xu <dxu@...uu.xyz>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Jiri Olsa <olsajiri@...il.com>, Quentin Monnet <quentin@...valent.com>, 
	Alan Maguire <alan.maguire@...cle.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, 
	Eddy Z <eddyz87@...il.com>, John Fastabend <john.fastabend@...il.com>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Kernel Team <kernel-team@...a.com>
Subject: Re: [PATCH bpf-next v4 08/12] bpf: verifier: Relax caller
 requirements for kfunc projection type args

On Mon, Jun 10, 2024 at 11:30:31AM GMT, Alexei Starovoitov wrote:
> On Sat, Jun 8, 2024 at 2:16 PM Daniel Xu <dxu@...uu.xyz> wrote:
> >
> > Currently, if a kfunc accepts a projection type as an argument (eg
> > struct __sk_buff *), the caller must exactly provide exactly the same
> > type with provable provenance.
> >
> > However in practice, kfuncs that accept projection types _must_ cast to
> > the underlying type before use b/c projection type layouts are
> > completely made up. Thus, it is ok to relax the verifier rules around
> > implicit conversions.
> >
> > We will use this functionality in the next commit when we align kfuncs
> > to user-facing types.
> >
> > Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> > ---
> >  kernel/bpf/verifier.c | 10 +++++++++-
> >  1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 81a3d2ced78d..0808beca3837 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -11257,6 +11257,8 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
> >         bool strict_type_match = false;
> >         const struct btf *reg_btf;
> >         const char *reg_ref_tname;
> > +       bool taking_projection;
> > +       bool struct_same;
> >         u32 reg_ref_id;
> >
> >         if (base_type(reg->type) == PTR_TO_BTF_ID) {
> > @@ -11300,7 +11302,13 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
> >
> >         reg_ref_t = btf_type_skip_modifiers(reg_btf, reg_ref_id, &reg_ref_id);
> >         reg_ref_tname = btf_name_by_offset(reg_btf, reg_ref_t->name_off);
> > -       if (!btf_struct_ids_match(&env->log, reg_btf, reg_ref_id, reg->off, meta->btf, ref_id, strict_type_match)) {
> > +       struct_same = btf_struct_ids_match(&env->log, reg_btf, reg_ref_id, reg->off, meta->btf, ref_id, strict_type_match);
> > +       /* If kfunc is accepting a projection type (ie. __sk_buff), it cannot
> > +        * actually use it -- it must cast to the underlying type. So we allow
> > +        * caller to pass in the underlying type.
> > +        */
> > +       taking_projection = !strcmp(ref_tname, "__sk_buff") && !strcmp(reg_ref_tname, "sk_buff");
> 
> xdp_md/buff probably as well?
> 
> And with that share the code with btf_is_prog_ctx_type() ?

Ack - will do.