CODEWITHSUNDEEP
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Sanito Gonzalez
Sanito Gonzalez

Reputation: 3

Compare BPF maps in fentry/htab_map_update_elem hook

I'd like to perform actions after a value is inserted to the specific map, using fentry/htab_map_update_elem hook. How can I check if struct bpf_map* map is the map that declared in BPF code?

For example,

// kernel BPF program

struct {
    __uint(type, BPF_MAP_TYPE_HASH);
    __uint(max_entries, 128);
    __type(key, __u32);
    __type(value, __u32);
} mymap SEC(".maps");

SEC("fentry/htab_map_update_elem")
int mymap_prog(struct bpf_map* map, void* key, void* value, u64 map_flags) {
    // Here, how to check if map == mymap?
}

// user program
int main() {
    struct mymap_bpf* skel;
    skel = ...

    // Update mymap and trigger map update hook
    bpf_map__update_elem(skel->maps.mymap, ...);

(Skeleton header file is generated with bpftool)

Thank you in advance.

Upvotes: 0

Views: 36

Answers (1)

pchaigno
pchaigno

Reputation: 13133

TL;DR. You can identify the map by checking its ID, with map->id and bpf_map_get_info_by_fd.

On the BPF side:

SEC("fentry/htab_map_update_elem")
int mymap_prog(struct bpf_map* map, void* key, void* value, u64 map_flags) {
    if (map->id == MY_MAP_ID)
        ...
}

On the libbpf side:

struct bpf_map_info info_m = {};

ret = bpf_map_get_info_by_fd(bpf_map__fd(skel->maps.mymap),
                             &info_m, sizeof(info_m));

// The map ID is at info_m.id.

Then you can either pass info_m.id to the BPF program via a map or you can hardcode it as MY_MAP_ID as I did above.

Upvotes: 0

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