On Wed, Mar 9, 2022 at 8:33 AM Dmitrii Dolgov <9erthalion6@xxxxxxxxx> wrote:
>
> 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>
> ---
Quentin, any opinion on this feature? The implementation seems
straightforward enough. We'll need to not forget to expand the support
to other types that support bpf_cookies (and multi-attach kprobes and
fentries will be problematic, potentially), but this might be useful
for debugging purposes.
> 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(+)
>
[...]
