tools api fs: More thread safety for global filesystem variables

Multiple threads, such as with "perf top", may race to initialize a
file system path like hugetlbfs. The racy initialization of the path
leads to at least memory leaks. To avoid this initialize each fs for
reading the mount point path with pthread_once.

Mounting the file system may also be racy, so introduce a mutex over
the function. This does mean that the path is being accessed with and
without a mutex, which is inherently racy but hopefully benign,
especially as there are fewer callers to fs__mount.

Remove the fs__entries by directly using global variables, this was
done as no argument like the index can be passed to the init once
routine.

Issue found and tested with "perf top" and address sanitizer.

Signed-off-by: Ian Rogers <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/lib/api/fs/fs.c

···
 // SPDX-License-Identifier: GPL-2.0
+#include <assert.h>
 #include <ctype.h>
 #include <errno.h>
 #include <limits.h>
···
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <fcntl.h>
+#include <pthread.h>
 #include <unistd.h>
 #include <sys/mount.h>
 
···
 #define BPF_FS_MAGIC           0xcafe4a11
 #endif
 
-static const char * const sysfs__fs_known_mountpoints[] = {
+static const char * const sysfs__known_mountpoints[] = {
 	"/sys",
 	0,
 };
···
 };
 
 struct fs {
-	const char		*name;
-	const char * const	*mounts;
+	const char *		 const name;
+	const char * const *	 const mounts;
 	char			*path;
-	bool			 found;
-	bool			 checked;
-	long			 magic;
-};
-
-enum {
-	FS__SYSFS   = 0,
-	FS__PROCFS  = 1,
-	FS__DEBUGFS = 2,
-	FS__TRACEFS = 3,
-	FS__HUGETLBFS = 4,
-	FS__BPF_FS = 5,
+	pthread_mutex_t		 mount_mutex;
+	const long		 magic;
 };
 
 #ifndef TRACEFS_MAGIC
 #define TRACEFS_MAGIC 0x74726163
 #endif
 
-static struct fs fs__entries[] = {
-	[FS__SYSFS] = {
-		.name	= "sysfs",
-		.mounts	= sysfs__fs_known_mountpoints,
-		.magic	= SYSFS_MAGIC,
-		.checked = false,
-	},
-	[FS__PROCFS] = {
-		.name	= "proc",
-		.mounts	= procfs__known_mountpoints,
-		.magic	= PROC_SUPER_MAGIC,
-		.checked = false,
-	},
-	[FS__DEBUGFS] = {
-		.name	= "debugfs",
-		.mounts	= debugfs__known_mountpoints,
-		.magic	= DEBUGFS_MAGIC,
-		.checked = false,
-	},
-	[FS__TRACEFS] = {
-		.name	= "tracefs",
-		.mounts	= tracefs__known_mountpoints,
-		.magic	= TRACEFS_MAGIC,
-		.checked = false,
-	},
-	[FS__HUGETLBFS] = {
-		.name	= "hugetlbfs",
-		.mounts = hugetlbfs__known_mountpoints,
-		.magic	= HUGETLBFS_MAGIC,
-		.checked = false,
-	},
-	[FS__BPF_FS] = {
-		.name	= "bpf",
-		.mounts = bpf_fs__known_mountpoints,
-		.magic	= BPF_FS_MAGIC,
-		.checked = false,
-	},
-};
+static void fs__init_once(struct fs *fs);
+static const char *fs__mountpoint(const struct fs *fs);
+static const char *fs__mount(struct fs *fs);
+
+#define FS(lower_name, fs_name, upper_name)		\
+static struct fs fs__##lower_name = {			\
+	.name = #fs_name,				\
+	.mounts = lower_name##__known_mountpoints,	\
+	.magic = upper_name##_MAGIC,			\
+	.mount_mutex = PTHREAD_MUTEX_INITIALIZER,	\
+};							\
+							\
+static void lower_name##_init_once(void)		\
+{							\
+	struct fs *fs = &fs__##lower_name;		\
+							\
+	fs__init_once(fs);				\
+}							\
+							\
+const char *lower_name##__mountpoint(void)		\
+{							\
+	static pthread_once_t init_once = PTHREAD_ONCE_INIT;	\
+	struct fs *fs = &fs__##lower_name;		\
+							\
+	pthread_once(&init_once, lower_name##_init_once);	\
+	return fs__mountpoint(fs);			\
+}							\
+							\
+const char *lower_name##__mount(void)			\
+{							\
+	const char *mountpoint = lower_name##__mountpoint();	\
+	struct fs *fs = &fs__##lower_name;		\
+							\
+	if (mountpoint)					\
+		return mountpoint;			\
+							\
+	return fs__mount(fs);				\
+}							\
+							\
+bool lower_name##__configured(void)			\
+{							\
+	return lower_name##__mountpoint() != NULL;	\
+}
+
+FS(sysfs, sysfs, SYSFS);
+FS(procfs, procfs, PROC_SUPER);
+FS(debugfs, debugfs, DEBUGFS);
+FS(tracefs, tracefs, TRACEFS);
+FS(hugetlbfs, hugetlbfs, HUGETLBFS);
+FS(bpf_fs, bpf, BPF_FS);
 
 static bool fs__read_mounts(struct fs *fs)
 {
-	bool found = false;
 	char type[100];
 	FILE *fp;
 	char path[PATH_MAX + 1];
···
 	if (fp == NULL)
 		return false;
 
-	while (!found &&
-	       fscanf(fp, "%*s %" STR(PATH_MAX) "s %99s %*s %*d %*d\n",
+	while (fscanf(fp, "%*s %" STR(PATH_MAX) "s %99s %*s %*d %*d\n",
 		      path, type) == 2) {
 
 		if (strcmp(type, fs->name) == 0) {
-			free(fs->path);
 			fs->path = strdup(path);
-			if (!fs->path)
-				return false;
-			found = true;
+			fclose(fp);
+			return fs->path != NULL;
 		}
 	}
-
 	fclose(fp);
-	fs->checked = true;
-	return fs->found = found;
+	return false;
 }
 
 static int fs__valid_mount(const char *fs, long magic)
···
 	ptr = fs->mounts;
 	while (*ptr) {
 		if (fs__valid_mount(*ptr, fs->magic) == 0) {
-			free(fs->path);
 			fs->path = strdup(*ptr);
 			if (!fs->path)
 				return false;
-			fs->found = true;
 			return true;
 		}
 		ptr++;
···
 	if (!override_path)
 		return false;
 
-	free(fs->path);
 	fs->path = strdup(override_path);
 	if (!fs->path)
 		return false;
-	fs->found = true;
-	fs->checked = true;
 	return true;
 }
 
-static const char *fs__get_mountpoint(struct fs *fs)
+static void fs__init_once(struct fs *fs)
 {
-	if (fs__env_override(fs))
-		return fs->path;
-
-	if (fs__check_mounts(fs))
-		return fs->path;
-
-	if (fs__read_mounts(fs))
-		return fs->path;
-
-	return NULL;
+	if (!fs__env_override(fs) &&
+	    !fs__check_mounts(fs) &&
+	    !fs__read_mounts(fs)) {
+		assert(!fs->path);
+	} else {
+		assert(fs->path);
+	}
 }
 
-static const char *fs__mountpoint(int idx)
+static const char *fs__mountpoint(const struct fs *fs)
 {
-	struct fs *fs = &fs__entries[idx];
-
-	if (fs->found)
-		return (const char *)fs->path;
-
-	/* the mount point was already checked for the mount point
-	 * but and did not exist, so return NULL to avoid scanning again.
-	 * This makes the found and not found paths cost equivalent
-	 * in case of multiple calls.
-	 */
-	if (fs->checked)
-		return NULL;
-
-	return fs__get_mountpoint(fs);
+	return fs->path;
 }
 
 static const char *mount_overload(struct fs *fs)
···
 	return getenv(upper_name) ?: *fs->mounts;
 }
 
-static const char *fs__mount(int idx)
+static const char *fs__mount(struct fs *fs)
 {
-	struct fs *fs = &fs__entries[idx];
 	const char *mountpoint;
 
-	if (fs__mountpoint(idx))
-		return (const char *)fs->path;
+	pthread_mutex_lock(&fs->mount_mutex);
+
+	/* Check if path found inside the mutex to avoid races with other callers of mount. */
+	mountpoint = fs__mountpoint(fs);
+	if (mountpoint)
+		goto out;
 
 	mountpoint = mount_overload(fs);
 
-	if (mount(NULL, mountpoint, fs->name, 0, NULL) < 0)
-		return NULL;
-
-	return fs__check_mounts(fs) ? fs->path : NULL;
+	if (mount(NULL, mountpoint, fs->name, 0, NULL) == 0 &&
+	    fs__valid_mount(mountpoint, fs->magic) == 0) {
+		fs->path = strdup(mountpoint);
+		mountpoint = fs->path;
+	}
+out:
+	pthread_mutex_unlock(&fs->mount_mutex);
+	return mountpoint;
 }
-
-#define FS(name, idx)				\
-const char *name##__mountpoint(void)		\
-{						\
-	return fs__mountpoint(idx);		\
-}						\
-						\
-const char *name##__mount(void)			\
-{						\
-	return fs__mount(idx);			\
-}						\
-						\
-bool name##__configured(void)			\
-{						\
-	return name##__mountpoint() != NULL;	\
-}
-
-FS(sysfs,   FS__SYSFS);
-FS(procfs,  FS__PROCFS);
-FS(debugfs, FS__DEBUGFS);
-FS(tracefs, FS__TRACEFS);
-FS(hugetlbfs, FS__HUGETLBFS);
-FS(bpf_fs, FS__BPF_FS);
 
 int filename__read_int(const char *filename, int *value)
 {