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 [off-list ref] wrote:

On 01/21/2019 10:10 AM, Maciej Fijalkowski wrote:

quoted

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 <redacted>
Signed-off-by: Maciej Fijalkowski <redacted>
Reviewed-by: Jakub Kicinski <redacted>
---
 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.

quoted

+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.

quoted

+}
+
 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;