libsensors: Avoid non-symlink sysfs device entries

Devices under the MMC / SDIO bus have a "device" entry that is not a
symbolic link; instead it contains the device ID of the SDIO device.
This causes the sysfs parsing code in libsensors to error out, as it
expects all "device" entries be symbolic links to the underlying device.

Check for non-symlink entries before calling realpath() to avoid using
it as a valid device link.

Signed-off-by: Chen-Yu Tsai <wenst@google.com>

lib/sysfs.c

@@ -700,6 +700,26 @@ static int classify_device(const char *dev_name,
 	return ret;
 }
 
+/* realpath(3) that returns NULL for a path that isn't a symlink.
+   This is needed to deal with devices that have a "device" sysfs
+   entry that has the device ID, such as SDIO devices. */
+static char* realpath_no_symlink(const char *restrict path,
+				 char *restrict resolved_path)
+{
+	struct stat st;
+	int ret;
+
+	if (lstat(path, &st) < 0)
+		return NULL;
+
+	if (!S_ISLNK(st.st_mode)) {
+		errno = -EINVAL;
+		return NULL;
+	}
+
+	return realpath(path, resolved_path);
+}
+
 static int find_bus_type(const char *dev_path,
                          const char *dev_name,
                          sensors_chip_features *entry)
@@ -740,7 +760,7 @@ static int find_bus_type(const char *dev_path,
 		if (!ret) {
 			snprintf(linkpath, NAME_MAX, "%s/device", my_dev_path);
 			free(my_dev_path);
-			my_dev_path = realpath(linkpath, NULL);
+			my_dev_path = realpath_no_symlink(linkpath, NULL);
 			if (my_dev_path != NULL)
 				dev_name = strrchr(my_dev_path, '/') + 1;
 			else if (errno == ENOMEM)
@@ -838,7 +858,7 @@ static int sensors_add_hwmon_device(const char *path, const char *classdev)
 	(void)classdev; /* hide warning */
 
 	snprintf(linkpath, NAME_MAX, "%s/device", path);
-	dev_path = realpath(linkpath, NULL);
+	dev_path = realpath_no_symlink(linkpath, NULL);
 	if (dev_path == NULL) {
 		if (errno == ENOMEM) {
 			sensors_fatal_error(__func__, "Out of memory");