netdev - Re: [PATCH v4 bpf-next 06/14] bpf: Use BTF_ID to resolve bpf_ctx

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Date:   Sun, 28 Jun 2020 21:52:54 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Yonghong Song <yhs@...com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Song Liu <songliubraving@...com>,
        Martin KaFai Lau <kafai@...com>,
        David Miller <davem@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Wenbo Zhang <ethercflow@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Andrii Nakryiko <andriin@...com>,
        Brendan Gregg <bgregg@...flix.com>,
        Florent Revest <revest@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v4 bpf-next 06/14] bpf: Use BTF_ID to resolve
 bpf_ctx_convert struct

On Fri, Jun 26, 2020 at 02:44:39PM -0700, Yonghong Song wrote:
> 
> 
> On 6/25/20 3:12 PM, Jiri Olsa wrote:
> > This way the ID is resolved during compile time,
> > and we can remove the runtime name search.
> > 
> > Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> > ---
> >   kernel/bpf/btf.c | 12 ++++++++----
> >   1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index 4da6b0770ff9..701a2cb5dfb2 100644
> > --- a/kernel/bpf/btf.c
> > +++ b/kernel/bpf/btf.c
> > @@ -18,6 +18,7 @@
> >   #include <linux/sort.h>
> >   #include <linux/bpf_verifier.h>
> >   #include <linux/btf.h>
> > +#include <linux/btf_ids.h>
> >   #include <linux/skmsg.h>
> >   #include <linux/perf_event.h>
> >   #include <net/sock.h>
> > @@ -3621,6 +3622,9 @@ static int btf_translate_to_vmlinux(struct bpf_verifier_log *log,
> >   	return kern_ctx_type->type;
> >   }
> > +BTF_ID_LIST(bpf_ctx_convert_btf_id)
> > +BTF_ID(struct, bpf_ctx_convert)
> > +
> >   struct btf *btf_parse_vmlinux(void)
> >   {
> >   	struct btf_verifier_env *env = NULL;
> > @@ -3659,10 +3663,10 @@ struct btf *btf_parse_vmlinux(void)
> >   	if (err)
> >   		goto errout;
> > -	/* find struct bpf_ctx_convert for type checking later */
> > -	btf_id = btf_find_by_name_kind(btf, "bpf_ctx_convert", BTF_KIND_STRUCT);
> > -	if (btf_id < 0) {
> > -		err = btf_id;
> > +	/* struct bpf_ctx_convert for type checking later */
> > +	btf_id = bpf_ctx_convert_btf_id[0];
> > +	if (btf_id <= 0) {
> 
> Just want to double check. Is it possible btf_id < 0 since previous patch
> did not check < 0?

right.. resolve_btfids would fail during linking,
so this is not necessary

thanks,
jirka