2022-03-09 17:31 UTC+0100 ~ Dmitrii Dolgov <9erthalion6@xxxxxxxxx>
> Commit 82e6b1eee6a8 ("bpf: Allow to specify user-provided bpf_cookie for
> BPF perf links") introduced the concept of user specified bpf_cookie,
> which could be accessed by BPF programs using bpf_get_attach_cookie().
> For troubleshooting purposes it is convenient to expose bpf_cookie via
> bpftool as well, so there is no need to meddle with the target BPF
> program itself.
>
> Implemented using the pid iterator BPF program to actually fetch
> bpf_cookies, which allows constraining code changes only to bpftool.
>
> $ bpftool link
> 1: type 7 prog 5
> bpf_cookie 123
> pids bootstrap(81)
>
> Signed-off-by: Dmitrii Dolgov <9erthalion6@xxxxxxxxx>
> Acked-by: Yonghong Song <yhs@xxxxxx>
> ---
> Changes in v6:
> - Remove unnecessary initialization of fields in pid_iter_entry
> - Changing bpf_cookie_set to has_bpf_cookie
> - Small code cleanup (casting bpf_cookie when needed, removing
> __always_inline, etc.)
>
> Changes in v5:
> - Remove unneeded cookie assigns
>
> Changes in v4:
> - Fetch cookies only for bpf_perf_link
> - Signal about bpf_cookie via the flag, instead of deducing it from
> the object and link type
> - Reset pid_iter_entry to avoid invalid indirect read from stack
>
> Changes in v3:
> - Use pid iterator to fetch bpf_cookie
>
> Changes in v2:
> - Display bpf_cookie in bpftool link command instead perf
>
> Previous discussion: https://lore.kernel.org/bpf/20220225152802.20957-1-9erthalion6@xxxxxxxxx/
>
> tools/bpf/bpftool/main.h | 2 ++
> tools/bpf/bpftool/pids.c | 8 ++++++++
> tools/bpf/bpftool/skeleton/pid_iter.bpf.c | 22 ++++++++++++++++++++++
> tools/bpf/bpftool/skeleton/pid_iter.h | 2 ++
> 4 files changed, 34 insertions(+)
>
> diff --git a/tools/bpf/bpftool/main.h b/tools/bpf/bpftool/main.h
> index 0c3840596b5a..3574bef7d4ce 100644
> --- a/tools/bpf/bpftool/main.h
> +++ b/tools/bpf/bpftool/main.h
> @@ -114,6 +114,8 @@ struct obj_ref {
> struct obj_refs {
> int ref_cnt;
> struct obj_ref *refs;
> + bool has_bpf_cookie;
> + __u64 bpf_cookie;
> };
>
> struct btf;
> diff --git a/tools/bpf/bpftool/pids.c b/tools/bpf/bpftool/pids.c
> index 7c384d10e95f..bb6c969a114a 100644
> --- a/tools/bpf/bpftool/pids.c
> +++ b/tools/bpf/bpftool/pids.c
> @@ -78,6 +78,8 @@ static void add_ref(struct hashmap *map, struct pid_iter_entry *e)
> ref->pid = e->pid;
> memcpy(ref->comm, e->comm, sizeof(ref->comm));
> refs->ref_cnt = 1;
> + refs->has_bpf_cookie = e->has_bpf_cookie;
> + refs->bpf_cookie = e->bpf_cookie;
>
> err = hashmap__append(map, u32_as_hash_field(e->id), refs);
> if (err)
> @@ -205,6 +207,9 @@ void emit_obj_refs_json(struct hashmap *map, __u32 id,
> if (refs->ref_cnt == 0)
> break;
>
> + if (refs->has_bpf_cookie)
> + jsonw_lluint_field(json_writer, "bpf_cookie", refs->bpf_cookie);
> +
Thinking again about this patch, shouldn't the JSON output for the
cookie(s) be an array if we expect to have several cookies for
multi-attach links in the future?
Quentin
