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

From: Jiri Olsa <[email-masked]>
Date: Tue, 7 Jan 2020 13:13:19 +0100

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 <[email-masked]>
> > ---
> >   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

jirka