perf list: Add scandirat compatibility function

scandirat is used during the printing of tracepoint events but may be
missing from certain libcs. Add a compatibility implementation that
uses the symlink of an fd in /proc as a path for the reliably present
scandir.
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Justin Stitt <justinstitt@google.com>
Cc: Bill Wendling <morbo@google.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Yang Jihong <yangjihong1@huawei.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: llvm@lists.linux.dev
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240221034155.1500118-3-irogers@google.com

tools/perf/util/print-events.c

@@ -28,6 +28,7 @@
 #include "tracepoint.h"
 #include "pfm.h"
 #include "thread_map.h"
+#include "util.h"
 
 #define MAX_NAME_LEN 100
 
@@ -63,6 +64,8 @@ void print_tracepoint_events(const struct print_callbacks *print_cb __maybe_unus
 {
 	char *events_path = get_tracing_file("events");
 	int events_fd = open(events_path, O_PATH);
+	struct dirent **sys_namelist = NULL;
+	int sys_items;
 
 	put_tracing_file(events_path);
 	if (events_fd < 0) {
@@ -70,10 +73,7 @@ void print_tracepoint_events(const struct print_callbacks *print_cb __maybe_unus
 		return;
 	}
 
-#ifdef HAVE_SCANDIRAT_SUPPORT
-{
-	struct dirent **sys_namelist = NULL;
-	int sys_items = tracing_events__scandir_alphasort(&sys_namelist);
+	sys_items = tracing_events__scandir_alphasort(&sys_namelist);
 
 	for (int i = 0; i < sys_items; i++) {
 		struct dirent *sys_dirent = sys_namelist[i];
@@ -130,11 +130,6 @@ void print_tracepoint_events(const struct print_callbacks *print_cb __maybe_unus
 	}
 
 	free(sys_namelist);
-}
-#else
-	printf("\nWARNING: Your libc doesn't have the scandirat function, please ask its maintainers to implement it.\n"
-	       "         As a rough fallback, please do 'ls %s' to see the available tracepoint events.\n", events_path);
-#endif
 	close(events_fd);
 }
 

tools/perf/util/util.c

@@ -552,3 +552,22 @@ int sched_getcpu(void)
 	return -1;
 }
 #endif
+
+#ifndef HAVE_SCANDIRAT_SUPPORT
+int scandirat(int dirfd, const char *dirp,
+	      struct dirent ***namelist,
+	      int (*filter)(const struct dirent *),
+	      int (*compar)(const struct dirent **, const struct dirent **))
+{
+	char path[PATH_MAX];
+	int err, fd = openat(dirfd, dirp, O_PATH);
+
+	if (fd < 0)
+		return fd;
+
+	snprintf(path, sizeof(path), "/proc/%d/fd/%d", getpid(), fd);
+	err = scandir(path, namelist, filter, compar);
+	close(fd);
+	return err;
+}
+#endif

tools/perf/util/util.h

@@ -6,6 +6,7 @@
 /* glibc 2.20 deprecates _BSD_SOURCE in favour of _DEFAULT_SOURCE */
 #define _DEFAULT_SOURCE 1
 
+#include <dirent.h>
 #include <fcntl.h>
 #include <stdbool.h>
 #include <stddef.h>
@@ -56,6 +57,13 @@ int perf_tip(char **strp, const char *dirpath);
 int sched_getcpu(void);
 #endif
 
+#ifndef HAVE_SCANDIRAT_SUPPORT
+int scandirat(int dirfd, const char *dirp,
+	      struct dirent ***namelist,
+	      int (*filter)(const struct dirent *),
+	      int (*compar)(const struct dirent **, const struct dirent **));
+#endif
+
 extern bool perf_singlethreaded;
 
 void perf_set_singlethreaded(void);