sd-device: introduce device_get_sysattr_safe_string()

This introduce a helper function that verifies read sysattr value.
Some sysattr value may come from firmware, and kernel may expose unsafe
string. The introduced function should be used when we use the result as
a string.

src/libsystemd/sd-device/device-private.h

@@ -16,6 +16,7 @@ int device_get_property_bool(sd_device *device, const char *key);
 int device_get_property_int(sd_device *device, const char *key, int *ret);
 int device_get_property_uint(sd_device *device, const char *key, unsigned *ret);
 int device_get_ifname(sd_device *device, const char **ret);
+int device_get_sysattr_safe_string(sd_device *device, const char *sysattr, const char **ret);
 int device_get_sysattr_int(sd_device *device, const char *sysattr, int *ret_value);
 int device_get_sysattr_unsigned_full(sd_device *device, const char *sysattr, unsigned base, unsigned *ret_value);
 static inline int device_get_sysattr_unsigned(sd_device *device, const char *sysattr, unsigned *ret_value) {

src/libsystemd/sd-device/sd-device.c

@@ -2619,6 +2619,34 @@ _public_ int sd_device_get_sysattr_value(sd_device *device, const char *sysattr,
         return sd_device_get_sysattr_value_with_size(device, sysattr, ret_value, NULL);
 }
 
+int device_get_sysattr_safe_string(sd_device *device, const char *sysattr, const char **ret) {
+        const char *value;
+        int r;
+
+        r = sd_device_get_sysattr_value(device, sysattr, &value);
+        if (r < 0)
+                return r;
+
+        if (!string_is_safe(value,
+                            STRING_ALLOW_EMPTY |
+                            STRING_ALLOW_NEWLINES |
+                            STRING_ALLOW_BACKSLASHES |
+                            STRING_ALLOW_QUOTES |
+                            STRING_ALLOW_GLOBS)) {
+                if (DEBUG_LOGGING) {
+                        _cleanup_free_ char *escaped = cescape(value);
+                        log_device_debug(device, "sd-device: '%s' sysattr contains invalid characters, refusing: %s",
+                                         sysattr, strnull(escaped));
+                }
+                return -ENXIO;
+        }
+
+        if (ret)
+                *ret = value;
+
+        return 0;
+}
+
 int device_get_sysattr_int(sd_device *device, const char *sysattr, int *ret_value) {
         const char *value;
         int r;

src/libsystemd/sd-device/test-sd-device.c

@@ -305,6 +305,13 @@ static void test_sd_device_one(sd_device *d) {
                 ASSERT_OK(r = device_get_sysattr_unsigned(d, "nsid", &x));
                 ASSERT_EQ(x > 0, r > 0);
         }
+
+        const char *uevent;
+        if (sd_device_get_sysattr_value(d, "uevent", &uevent) >= 0) {
+                const char *uevent_safe;
+                ASSERT_OK(device_get_sysattr_safe_string(d, "uevent", &uevent_safe));
+                ASSERT_STREQ(uevent, uevent_safe);
+        }
 }
 
 static void exclude_problematic_devices(sd_device_enumerator *e) {