Introduce several more helper functions to read sysfs attributes and use them where applicable (#41065)

Let's not use sd_device_get_sysattr_value() unless the result is
sanitized, escaped, or filtered.

src/basic/basic-forward.h

@@ -103,6 +103,7 @@ typedef enum Glyph Glyph;
 typedef enum ImageClass ImageClass;
 typedef enum JobMode JobMode;
 typedef enum RuntimeScope RuntimeScope;
+typedef enum StringSafeFlags StringSafeFlags;
 typedef enum TimestampStyle TimestampStyle;
 typedef enum UnitActiveState UnitActiveState;
 typedef enum UnitDependency UnitDependency;

src/basic/string-util.c

@@ -1113,7 +1113,11 @@ bool string_is_safe(const char *p, StringSafeFlags flags) {
                 return false;
 
         for (const char *t = p; *t; t++) {
-                if ((*t > 0 && *t < ' ') || *t == 0x7f) /* never allow control characters */
+                /* never allow control characters, except for new line */
+                if ((*t > 0 && *t < ' ' && *t != '\n') || *t == 0x7f)
+                        return false;
+
+                if (!FLAGS_SET(flags, STRING_ALLOW_NEWLINES) && *t == '\n')
                         return false;
 
                 if (!FLAGS_SET(flags, STRING_ALLOW_BACKSLASHES) && *t == '\\')

src/basic/string-util.h

@@ -223,10 +223,11 @@ static inline int strdup_to(char **ret, const char *src) {
 typedef enum StringSafeFlags {
         STRING_ASCII             = 1 << 0, /* Verify string is 7-Bit ASCII (rather than just UTF-8) */
         STRING_ALLOW_EMPTY       = 1 << 1, /* Allow empty strings */
-        STRING_ALLOW_BACKSLASHES = 1 << 2, /* Allow backslashes (\) */
-        STRING_ALLOW_QUOTES      = 1 << 3, /* Allow quotes (" or ') */
-        STRING_ALLOW_GLOBS       = 1 << 4, /* Allow globs (?, * or [) */
-        STRING_FILENAME          = 1 << 5, /* Verify the string is valid as regular filename */
+        STRING_ALLOW_NEWLINES    = 1 << 2, /* Allow newlines (\n) */
+        STRING_ALLOW_BACKSLASHES = 1 << 3, /* Allow backslashes (\) */
+        STRING_ALLOW_QUOTES      = 1 << 4, /* Allow quotes (" or ') */
+        STRING_ALLOW_GLOBS       = 1 << 5, /* Allow globs (?, * or [) */
+        STRING_FILENAME          = 1 << 6, /* Verify the string is valid as regular filename */
 } StringSafeFlags;
 
 bool string_is_safe(const char *p, StringSafeFlags flags) _pure_;