Re: [PATCH 1/5] bpf: Allow non struct type for btf ctx access

[Jiri Olsa <jolsa@redhat.com>]

On Tue, Jan 07, 2020 at 01:13:23PM +0100, Jiri Olsa wrote:
> On Mon, Jan 06, 2020 at 09:36:17PM +0000, Yonghong Song wrote:
> > 
> > 
> > On 12/29/19 6:37 AM, Jiri Olsa wrote:
> > > I'm not sure why the restriction was added,
> > > but I can't access pointers to POD types like
> > > const char * when probing vfs_read function.
> > > 
> > > Removing the check and allow non struct type
> > > access in context.
> > > 
> > > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > > ---
> > >   kernel/bpf/btf.c | 6 ------
> > >   1 file changed, 6 deletions(-)
> > > 
> > > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > > index ed2075884724..ae90f60ac1b8 100644
> > > --- a/kernel/bpf/btf.c
> > > +++ b/kernel/bpf/btf.c
> > > @@ -3712,12 +3712,6 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> > >   	/* skip modifiers */
> > >   	while (btf_type_is_modifier(t))
> > >   		t = btf_type_by_id(btf, t->type);
> > > -	if (!btf_type_is_struct(t)) {
> > > -		bpf_log(log,
> > > -			"func '%s' arg%d type %s is not a struct\n",
> > > -			tname, arg, btf_kind_str[BTF_INFO_KIND(t->info)]);
> > > -		return false;
> > > -	}
> > 
> > Hi, Jiri, the RFC looks great! Especially, you also referenced this will
> > give great performance boost for bcc scripts.
> > 
> > Could you provide more context on why the above change is needed?
> > The function btf_ctx_access is used to check validity of accessing
> > function parameters which are wrapped inside a structure, I am wondering
> > what kinds of accesses you tried to address here.
> 
> when I was transforming opensnoop.py to use this I got fail in
> there when I tried to access filename arg in do_sys_open
> 
> but actualy it seems this should get recognized earlier by:
> 
>           if (btf_type_is_int(t))
>                 /* accessing a scalar */
>                 return true;
> 
> I'm not sure why it did not pass for const char*, I'll check

it seems we don't check for pointer to scalar (just void),
which is the case in my example 'const char *filename'

I'll post this in v2 with other changes

jirka


---
diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index ed2075884724..650df4ed346e 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3633,7 +3633,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		    const struct bpf_prog *prog,
 		    struct bpf_insn_access_aux *info)
 {
-	const struct btf_type *t = prog->aux->attach_func_proto;
+	const struct btf_type *tp, *t = prog->aux->attach_func_proto;
 	struct bpf_prog *tgt_prog = prog->aux->linked_prog;
 	struct btf *btf = bpf_prog_get_target_btf(prog);
 	const char *tname = prog->aux->attach_func_name;
@@ -3695,6 +3695,17 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		 */
 		return true;
 
+	tp = btf_type_by_id(btf, t->type);
+	/* skip modifiers */
+	while (btf_type_is_modifier(tp))
+		tp = btf_type_by_id(btf, tp->type);
+
+	if (btf_type_is_int(tp))
+		/* This is a pointer scalar.
+		 * It is the same as scalar from the verifier safety pov.
+		 */
+		return true;
+
 	/* this is a pointer to another type */
 	info->reg_type = PTR_TO_BTF_ID;
 	info->btf_id = t->type;