[PATCH bpf-next 1/4] bpftool: use a local copy of perf_event to fix accessing ::bpf_cookie

[Quentin Monnet <quentin@xxxxxxxxxxxxx>]

From: Alexander Lobakin <alobakin@xxxxx>

When CONFIG_PERF_EVENTS is not set, struct perf_event remains empty.
However, the structure is being used by bpftool indirectly via BTF.
This leads to:

skeleton/pid_iter.bpf.c:49:30: error: no member named 'bpf_cookie' in 'struct perf_event'
        return BPF_CORE_READ(event, bpf_cookie);
               ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~

...

skeleton/pid_iter.bpf.c:49:9: error: returning 'void' from a function with incompatible result type '__u64' (aka 'unsigned long long')
        return BPF_CORE_READ(event, bpf_cookie);
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Tools and samples can't use any CONFIG_ definitions, so the fields
used there should always be present.
Define struct perf_event___local with the `preserve_access_index`
attribute inside the pid_iter BPF prog to allow compiling on any
configs. CO-RE will substitute it with the real struct perf_event
accesses later on.

Fixes: cbdaf71f7e65 ("bpftool: Add bpf_cookie to link output")
Suggested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
Signed-off-by: Alexander Lobakin <alobakin@xxxxx>
Signed-off-by: Quentin Monnet <quentin@xxxxxxxxxxxxx>
---
 tools/bpf/bpftool/skeleton/pid_iter.bpf.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/bpf/bpftool/skeleton/pid_iter.bpf.c b/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
index eb05ea53afb1..e2af8e5fb29e 100644
--- a/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
+++ b/tools/bpf/bpftool/skeleton/pid_iter.bpf.c
@@ -15,6 +15,10 @@ enum bpf_obj_type {
 	BPF_OBJ_BTF,
 };
 
+struct perf_event___local {
+	u64 bpf_cookie;
+} __attribute__((preserve_access_index));
+
 extern const void bpf_link_fops __ksym;
 extern const void bpf_map_fops __ksym;
 extern const void bpf_prog_fops __ksym;
@@ -41,8 +45,8 @@ static __always_inline __u32 get_obj_id(void *ent, enum bpf_obj_type type)
 /* could be used only with BPF_LINK_TYPE_PERF_EVENT links */
 static __u64 get_bpf_cookie(struct bpf_link *link)
 {
+	struct perf_event___local *event;
 	struct bpf_perf_link *perf_link;
-	struct perf_event *event;
 
 	perf_link = container_of(link, struct bpf_perf_link, link);
 	event = BPF_CORE_READ(perf_link, perf_file, private_data);
-- 
2.34.1