perf record: collect BPF metadata from existing BPF programs

Look for .rodata maps, find ones with 'bpf_metadata_' variables, extract
their values as strings, and create a new PERF_RECORD_BPF_METADATA
synthetic event using that data. The code gets invoked from the existing
routine perf_event__synthesize_one_bpf_prog().

For example, a BPF program with the following variables:

    const char bpf_metadata_version[] SEC(".rodata") = "3.14159";
    int bpf_metadata_value[] SEC(".rodata") = 42;

would generate a PERF_RECORD_BPF_METADATA record with:

    .prog_name        = <BPF program name, e.g. "bpf_prog_a1b2c3_foo">
    .nr_entries       = 2
    .entries[0].key   = "version"
    .entries[0].value = "3.14159"
    .entries[1].key   = "value"
    .entries[1].value = "42"

Each of the BPF programs and subprograms that share those variables would
get a distinct PERF_RECORD_BPF_METADATA record, with the ".prog_name"
showing the name of each program or subprogram. The prog_name is
deliberately the same as the ".name" field in the corresponding
PERF_RECORD_KSYMBOL record.

This code only gets invoked if support for displaying BTF char arrays
as strings is detected.

Signed-off-by: Blake Jones <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

Author: blakej11

tools/lib/perf/include/perf/event.h

@@ -467,6 +467,22 @@ struct perf_record_compressed2 {
 	char			 data[];
 };
 
+#define BPF_METADATA_KEY_LEN   64
+#define BPF_METADATA_VALUE_LEN 256
+#define BPF_PROG_NAME_LEN      KSYM_NAME_LEN
+
+struct perf_record_bpf_metadata_entry {
+	char key[BPF_METADATA_KEY_LEN];
+	char value[BPF_METADATA_VALUE_LEN];
+};
+
+struct perf_record_bpf_metadata {
+	struct perf_event_header	      header;
+	char				      prog_name[BPF_PROG_NAME_LEN];
+	__u64				      nr_entries;
+	struct perf_record_bpf_metadata_entry entries[];
+};
+
 enum perf_user_event_type { /* above any possible kernel type */
 	PERF_RECORD_USER_TYPE_START		= 64,
 	PERF_RECORD_HEADER_ATTR			= 64,
@@ -489,6 +505,7 @@ enum perf_user_event_type { /* above any possible kernel type */
 	PERF_RECORD_COMPRESSED			= 81,
 	PERF_RECORD_FINISHED_INIT		= 82,
 	PERF_RECORD_COMPRESSED2			= 83,
+	PERF_RECORD_BPF_METADATA		= 84,
 	PERF_RECORD_HEADER_MAX
 };
 
@@ -530,6 +547,7 @@ union perf_event {
 	struct perf_record_header_feature	feat;
 	struct perf_record_compressed		pack;
 	struct perf_record_compressed2		pack2;
+	struct perf_record_bpf_metadata		bpf_metadata;
 };
 
 #endif /* __LIBPERF_EVENT_H */