bpftool: Add bpf_cookie to link output

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@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220309163112.24141-1-9erthalion6@gmail.com

tools/bpftool: Show info for processes holding BPF map/prog/link/btf FDs

Add bpf_iter-based way to find all the processes that hold open FDs against
BPF object (map, prog, link, btf). bpftool always attempts to discover this,
but will silently give up if kernel doesn't yet support bpf_iter BPF programs.
Process name and PID are emitted for each process (task group).

Sample output for each of 4 BPF objects:

$ sudo ./bpftool prog show
2694: cgroup_device tag 8c42dee26e8cd4c2 gpl
loaded_at 2020-06-16T15:34:32-0700 uid 0
xlated 648B jited 409B memlock 4096B
pids systemd(1)
2907: cgroup_skb name egress tag 9ad187367cf2b9e8 gpl
loaded_at 2020-06-16T18:06:54-0700 uid 0
xlated 48B jited 59B memlock 4096B map_ids 2436
btf_id 1202
pids test_progs(2238417), test_progs(2238445)

$ sudo ./bpftool map show
2436: array name test_cgr.bss flags 0x400
key 4B value 8B max_entries 1 memlock 8192B
btf_id 1202
pids test_progs(2238417), test_progs(2238445)
2445: array name pid_iter.rodata flags 0x480
key 4B value 4B max_entries 1 memlock 8192B
btf_id 1214 frozen
pids bpftool(2239612)

$ sudo ./bpftool link show
61: cgroup prog 2908
cgroup_id 375301 attach_type egress
pids test_progs(2238417), test_progs(2238445)
62: cgroup prog 2908
cgroup_id 375344 attach_type egress
pids test_progs(2238417), test_progs(2238445)

$ sudo ./bpftool btf show
1202: size 1527B prog_ids 2908,2907 map_ids 2436
pids test_progs(2238417), test_progs(2238445)
1242: size 34684B
pids bpftool(2258892)

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20200619231703.738941-9-andriin@fb.com