netdev - Re: [PATCH bpf-next v2 1/8] libbpf: Add a helper for retrieving a map fd for a given name

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Date:   Wed, 23 Jan 2019 15:03:50 +0100
From:   Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     ast@...nel.org, netdev@...r.kernel.org,
        jakub.kicinski@...ronome.com, brouer@...hat.com
Subject: Re: [PATCH bpf-next v2 1/8] libbpf: Add a helper for retrieving a
 map fd for a given name

On Wed, 23 Jan 2019 11:54:41 +0100
Daniel Borkmann <daniel@...earbox.net> wrote:

> On 01/21/2019 10:10 AM, Maciej Fijalkowski wrote:
> > XDP samples are mostly cooperating with eBPF maps through their file
> > descriptors. In case of a eBPF program that contains multiple maps it
> > might be tiresome to iterate through them and call bpf_map__fd for each
> > one. Add a helper mostly based on bpf_object__find_map_by_name, but
> > instead of returning the struct bpf_map pointer, return map fd.
> > 
> > Bump libbpf ABI version to 0.0.2.
> > 
> > Suggested-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> > Signed-off-by: Maciej Fijalkowski <maciejromanfijalkowski@...il.com>
> > Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
> > ---
> >  tools/lib/bpf/libbpf.c   | 12 ++++++++++++
> >  tools/lib/bpf/libbpf.h   |  3 +++
> >  tools/lib/bpf/libbpf.map |  4 ++++
> >  3 files changed, 19 insertions(+)
> > 
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 169e347c76f6..dc838bea403f 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -2840,6 +2840,18 @@ bpf_object__find_map_by_name(struct bpf_object *obj, const char *name)
> >  	return NULL;
> >  }  
> 
> Application could just do: bpf_map__fd(bpf_object__find_map_by_name(...)) or
> bpf_object__find_map_by_name(...)->fd as both are exposed via library, though
> I guess it may be okay to have a helper for it as it feels this might be needed
> in many cases.
>
> > +int
> > +bpf_object__find_map_fd_by_name(struct bpf_object *obj, const char *name)
> > +{
> > +	struct bpf_map *pos;
> > +
> > +	bpf_map__for_each(pos, obj) {
> > +		if (pos->name && !strcmp(pos->name, name))
> > +			return bpf_map__fd(pos);
> > +	}
> > +	return -ENOENT;  
> 
> Can we instead just do:
> 
> int
> bpf_object__find_map_fd_by_name(struct bpf_object *obj, const char *name)
> {
>         return bpf_map__fd(bpf_object__find_map_by_name(obj, name));
> }
>

Yes of course, I will send v3 once we figure out what's the best way to supply
the prog section name for xdp_redirect_cpu.

> > +}
> > +
> >  struct bpf_map *
> >  bpf_object__find_map_by_offset(struct bpf_object *obj, size_t offset)
> >  {
> > diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
> > index 5f68d7b75215..7f10d36abdde 100644
> > --- a/tools/lib/bpf/libbpf.h
> > +++ b/tools/lib/bpf/libbpf.h
> > @@ -264,6 +264,9 @@ struct bpf_map;
> >  LIBBPF_API struct bpf_map *
> >  bpf_object__find_map_by_name(struct bpf_object *obj, const char *name);
> >  
> > +LIBBPF_API int
> > +bpf_object__find_map_fd_by_name(struct bpf_object *obj, const char *name);
> > +
> >  /*
> >   * Get bpf_map through the offset of corresponding struct bpf_map_def
> >   * in the BPF object file.
> > diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
> > index cd02cd4e2cc3..7c59e4f64082 100644
> > --- a/tools/lib/bpf/libbpf.map
> > +++ b/tools/lib/bpf/libbpf.map
> > @@ -124,3 +124,7 @@ LIBBPF_0.0.1 {
> >  	local:
> >  		*;
> >  };
> > +LIBBPF_0.0.2 {
> > +	global:
> > +		bpf_object__find_map_fd_by_name;
> > +} LIBBPF_0.0.1;
> >   
>