perf bpf-filter: Improve error messages

The BPF filter needs libbpf/BPF-skeleton support and root privilege.
Add error messages to help users understand the problem easily.

When it's not build with BPF support (make BUILD_BPF_SKEL=0).

  $ sudo perf record -e cycles --filter "pid != 0" true
  Error: BPF filter is requested but perf is not built with BPF.
  	Please make sure to build with libbpf and BPF skeleton.

   Usage: perf record [<options>] [<command>]
      or: perf record [<options>] -- <command> [<options>]

          --filter <filter>
                            event filter

When it supports BPF but runs without root or CAP_BPF.  Note that it
also checks pinned BPF filters.

  $ perf record -e cycles --filter "pid != 0" -o /dev/null true
  Error: BPF filter only works for users with the CAP_BPF capability!
  	Please run 'perf record --setup-filter pin' as root first.

   Usage: perf record [<options>] [<command>]
      or: perf record [<options>] -- <command> [<options>]

          --filter <filter>
                            event filter

Reviewed-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250604174835.1852481-1-namhyung@kernel.org
Signed-off-by: Namhyung Kim <namhyung@kernel.org>

Author: namhyung

tools/perf/util/bpf-filter.c

@@ -52,6 +52,7 @@
 #include <internal/xyarray.h>
 #include <perf/threadmap.h>
 
+#include "util/cap.h"
 #include "util/debug.h"
 #include "util/evsel.h"
 #include "util/target.h"
@@ -618,11 +619,38 @@ struct perf_bpf_filter_expr *perf_bpf_filter_expr__new(enum perf_bpf_filter_term
 	return expr;
 }
 
+static bool check_bpf_filter_capable(void)
+{
+	bool used_root;
+
+	if (perf_cap__capable(CAP_BPF, &used_root))
+		return true;
+
+	if (!used_root) {
+		/* Check if root already pinned the filter programs and maps */
+		int fd = get_pinned_fd("filters");
+
+		if (fd >= 0) {
+			close(fd);
+			return true;
+		}
+	}
+
+	pr_err("Error: BPF filter only works for %s!\n"
+	       "\tPlease run 'perf record --setup-filter pin' as root first.\n",
+	       used_root ? "root" : "users with the CAP_BPF capability");
+
+	return false;
+}
+
 int perf_bpf_filter__parse(struct list_head *expr_head, const char *str)
 {
 	YY_BUFFER_STATE buffer;
 	int ret;
 
+	if (!check_bpf_filter_capable())
+		return -EPERM;
+
 	buffer = perf_bpf_filter__scan_string(str);
 
 	ret = perf_bpf_filter_parse(expr_head);

tools/perf/util/bpf-filter.h

@@ -5,6 +5,7 @@
 #include <linux/list.h>
 
 #include "bpf_skel/sample-filter.h"
+#include "util/debug.h"
 
 struct perf_bpf_filter_expr {
 	struct list_head list;
@@ -38,6 +39,8 @@ int perf_bpf_filter__unpin(void);
 static inline int perf_bpf_filter__parse(struct list_head *expr_head __maybe_unused,
 					 const char *str __maybe_unused)
 {
+	pr_err("Error: BPF filter is requested but perf is not built with BPF.\n"
+		"\tPlease make sure to build with libbpf and BPF skeleton.\n");
 	return -EOPNOTSUPP;
 }
 static inline int perf_bpf_filter__prepare(struct evsel *evsel __maybe_unused,

tools/perf/util/cap.c

@@ -7,7 +7,6 @@
 #include "debug.h"
 #include <errno.h>
 #include <string.h>
-#include <linux/capability.h>
 #include <sys/syscall.h>
 #include <unistd.h>
 

tools/perf/util/cap.h

@@ -3,6 +3,7 @@
 #define __PERF_CAP_H
 
 #include <stdbool.h>
+#include <linux/capability.h>
 
 /* For older systems */
 #ifndef CAP_SYSLOG
@@ -13,6 +14,10 @@
 #define CAP_PERFMON	38
 #endif
 
+#ifndef CAP_BPF
+#define CAP_BPF		39
+#endif
+
 /* Query if a capability is supported, used_root is set if the fallback root check was used. */
 bool perf_cap__capable(int cap, bool *used_root);