perf tools: Add perf_exe() helper to find perf binary

Also convert one existing user.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20190224153722.27020-9-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/header.c

@@ -527,17 +527,11 @@ static int write_event_desc(struct feat_fd *ff,
 static int write_cmdline(struct feat_fd *ff,
 			 struct perf_evlist *evlist __maybe_unused)
 {
-	char buf[MAXPATHLEN];
-	u32 n;
-	int i, ret;
+	char pbuf[MAXPATHLEN], *buf;
+	int i, ret, n;
 
 	/* actual path to perf binary */
-	ret = readlink("/proc/self/exe", buf, sizeof(buf) - 1);
-	if (ret <= 0)
-		return -1;
-
-	/* readlink() does not add null termination */
-	buf[ret] = '\0';
+	buf = perf_exe(pbuf, MAXPATHLEN);
 
 	/* account for binary path */
 	n = perf_env.nr_cmdline + 1;

tools/perf/util/util.c

@@ -568,3 +568,13 @@ const char *perf_tip(const char *dirpath)
 
 	return tip;
 }
+
+char *perf_exe(char *buf, int len)
+{
+	int n = readlink("/proc/self/exe", buf, len);
+	if (n > 0) {
+		buf[n] = 0;
+		return buf;
+	}
+	return strcpy(buf, "perf");
+}

tools/perf/util/util.h

@@ -77,6 +77,8 @@ extern bool perf_singlethreaded;
 void perf_set_singlethreaded(void);
 void perf_set_multithreaded(void);
 
+char *perf_exe(char *buf, int len);
+
 #ifndef O_CLOEXEC
 #ifdef __sparc__
 #define O_CLOEXEC      0x400000