bpftool: Use a local bpf_perf_event_value to fix accessing its fields

Fix the following error when building bpftool:

  CLANG   profiler.bpf.o
  CLANG   pid_iter.bpf.o
skeleton/profiler.bpf.c:18:21: error: invalid application of 'sizeof' to an incomplete type 'struct bpf_perf_event_value'
        __uint(value_size, sizeof(struct bpf_perf_event_value));
                           ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helpers.h:13:39: note: expanded from macro '__uint'
tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_helper_defs.h:7:8: note: forward declaration of 'struct bpf_perf_event_value'
struct bpf_perf_event_value;
       ^

struct bpf_perf_event_value is being used in the kernel only when
CONFIG_BPF_EVENTS is enabled, so it misses a BTF entry then.
Define struct bpf_perf_event_value___local with the
`preserve_access_index` attribute inside the pid_iter BPF prog to
allow compiling on any configs. It is a full mirror of a UAPI
structure, so is compatible both with and w/o CO-RE.
bpf_perf_event_read_value() requires a pointer of the original type,
so a cast is needed.

Fixes: 47c09d6a ("bpftool: Introduce "prog profile" command")
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230707095425.168126-5-quentin@isovalent.com

tools/bpf/bpftool/skeleton/profiler.bpf.c

@@ -4,6 +4,12 @@
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
+struct bpf_perf_event_value___local {
+	__u64 counter;
+	__u64 enabled;
+	__u64 running;
+} __attribute__((preserve_access_index));
+
 /* map of perf event fds, num_cpu * num_metric entries */
 struct {
 	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
@@ -15,14 +21,14 @@ struct {
 struct {
 	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
 	__uint(key_size, sizeof(u32));
-	__uint(value_size, sizeof(struct bpf_perf_event_value));
+	__uint(value_size, sizeof(struct bpf_perf_event_value___local));
 } fentry_readings SEC(".maps");
 
 /* accumulated readings */
 struct {
 	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
 	__uint(key_size, sizeof(u32));
-	__uint(value_size, sizeof(struct bpf_perf_event_value));
+	__uint(value_size, sizeof(struct bpf_perf_event_value___local));
 } accum_readings SEC(".maps");
 
 /* sample counts, one per cpu */
@@ -39,7 +45,7 @@ const volatile __u32 num_metric = 1;
 SEC("fentry/XXX")
 int BPF_PROG(fentry_XXX)
 {
-	struct bpf_perf_event_value *ptrs[MAX_NUM_MATRICS];
+	struct bpf_perf_event_value___local *ptrs[MAX_NUM_MATRICS];
 	u32 key = bpf_get_smp_processor_id();
 	u32 i;
 
@@ -53,10 +59,10 @@ int BPF_PROG(fentry_XXX)
 	}
 
 	for (i = 0; i < num_metric && i < MAX_NUM_MATRICS; i++) {
-		struct bpf_perf_event_value reading;
+		struct bpf_perf_event_value___local reading;
 		int err;
 
-		err = bpf_perf_event_read_value(&events, key, &reading,
+		err = bpf_perf_event_read_value(&events, key, (void *)&reading,
 						sizeof(reading));
 		if (err)
 			return 0;
@@ -68,14 +74,14 @@ int BPF_PROG(fentry_XXX)
 }
 
 static inline void
-fexit_update_maps(u32 id, struct bpf_perf_event_value *after)
+fexit_update_maps(u32 id, struct bpf_perf_event_value___local *after)
 {
-	struct bpf_perf_event_value *before, diff;
+	struct bpf_perf_event_value___local *before, diff;
 
 	before = bpf_map_lookup_elem(&fentry_readings, &id);
 	/* only account samples with a valid fentry_reading */
 	if (before && before->counter) {
-		struct bpf_perf_event_value *accum;
+		struct bpf_perf_event_value___local *accum;
 
 		diff.counter = after->counter - before->counter;
 		diff.enabled = after->enabled - before->enabled;
@@ -93,7 +99,7 @@ fexit_update_maps(u32 id, struct bpf_perf_event_value *after)
 SEC("fexit/XXX")
 int BPF_PROG(fexit_XXX)
 {
-	struct bpf_perf_event_value readings[MAX_NUM_MATRICS];
+	struct bpf_perf_event_value___local readings[MAX_NUM_MATRICS];
 	u32 cpu = bpf_get_smp_processor_id();
 	u32 i, zero = 0;
 	int err;
@@ -102,7 +108,8 @@ int BPF_PROG(fexit_XXX)
 	/* read all events before updating the maps, to reduce error */
 	for (i = 0; i < num_metric && i < MAX_NUM_MATRICS; i++) {
 		err = bpf_perf_event_read_value(&events, cpu + i * num_cpu,
-						readings + i, sizeof(*readings));
+						(void *)(readings + i),
+						sizeof(*readings));
 		if (err)
 			return 0;
 	}