perf cap: Tidy up and improve capability testing

Remove dependence on libcap. libcap is only used to query whether a
capability is supported, which is just 1 capget system call.

If the capget system call fails, fall back on root permission
checking. Previously if libcap fails then the permission is assumed
not present which may be pessimistic/wrong.

Add a used_root out argument to perf_cap__capable to say whether the
fall back root check was used. This allows the correct error message,
"root" vs "users with the CAP_PERFMON or CAP_SYS_ADMIN capability", to
be selected.

Tidy uses of perf_cap__capable so that tests aren't repeated if capget
isn't supported.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Changbin Du <changbin.du@huawei.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linux.dev>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Oliver Upton <oliver.upton@linux.dev>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240806220614.831914-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 14 insertions, 14 deletions

diff --git a/tools/perf/builtin-ftrace.c b/tools/perf/builtin-ftrace.c
index a615c405d98f..88a87bf387d2 100644
--- a/tools/perf/builtin-ftrace.c
+++ b/tools/perf/builtin-ftrace.c
@@ -63,20 +63,21 @@ static void ftrace__workload_exec_failed_signal(int signo __maybe_unused,
 	done = true;
 }
 
-static int check_ftrace_capable(void)
+static bool check_ftrace_capable(void)
 {
-	if (!(perf_cap__capable(CAP_PERFMON) ||
-	      perf_cap__capable(CAP_SYS_ADMIN))) {
-		pr_err("ftrace only works for %s!\n",
-#ifdef HAVE_LIBCAP_SUPPORT
-		"users with the CAP_PERFMON or CAP_SYS_ADMIN capability"
-#else
-		"root"
-#endif
+	bool used_root;
+
+	if (perf_cap__capable(CAP_PERFMON, &used_root))
+		return true;
+
+	if (!used_root && perf_cap__capable(CAP_SYS_ADMIN, &used_root))
+		return true;
+
+	pr_err("ftrace only works for %s!\n",
+		used_root ? "root"
+			  : "users with the CAP_PERFMON or CAP_SYS_ADMIN capability"
 		);
-		return -1;
-	}
-	return 0;
+	return false;
 }
 
 static int __write_tracing_file(const char *name, const char *val, bool append)
@@ -1579,8 +1580,7 @@ int cmd_ftrace(int argc, const char **argv)
 	signal(SIGCHLD, sig_handler);
 	signal(SIGPIPE, sig_handler);
 
-	ret = check_ftrace_capable();
-	if (ret < 0)
+	if (!check_ftrace_capable())
 		return -1;
 
 	ret = perf_config(perf_ftrace_config, &ftrace);