efivars: add helper that reads an fs path from an efi var

src/basic/efivars.c

@@ -145,8 +145,10 @@ int efi_get_variable(
 int efi_get_variable_string(const char *variable, char **ret) {
         _cleanup_free_ void *s = NULL;
         size_t ss = 0;
-        int r;
         char *x;
+        int r;
+
+        assert(variable);
 
         r = efi_get_variable(variable, NULL, &s, &ss);
         if (r < 0)
@@ -156,10 +158,27 @@ int efi_get_variable_string(const char *variable, char **ret) {
         if (!x)
                 return -ENOMEM;
 
-        *ret = x;
+        if (ret)
+                *ret = x;
+
         return 0;
 }
 
+int efi_get_variable_path(const char *variable, char **ret) {
+        int r;
+
+        assert(variable);
+
+        r = efi_get_variable_string(variable, ret);
+        if (r < 0)
+                return r;
+
+        if (ret)
+                efi_tilt_backslashes(*ret);
+
+        return r;
+}
+
 static int efi_verify_variable(const char *variable, uint32_t attr, const void *value, size_t size) {
         _cleanup_free_ void *buf = NULL;
         size_t n;

src/basic/efivars.h

@@ -11,6 +11,7 @@
 #include "sd-id128.h"
 
 #include "efivars-fundamental.h"
+#include "string-util.h"
 #include "time-util.h"
 
 #define EFI_VENDOR_LOADER       SD_ID128_MAKE(4a,67,b0,82,0a,4c,41,cf,b6,c7,44,0b,29,bb,8c,4f)
@@ -47,6 +48,7 @@
 
 int efi_get_variable(const char *variable, uint32_t *attribute, void **ret_value, size_t *ret_size);
 int efi_get_variable_string(const char *variable, char **ret);
+int efi_get_variable_path(const char *variable, char **ret);
 int efi_set_variable(const char *variable, const void *value, size_t size);
 int efi_set_variable_string(const char *variable, const char *p);
 
@@ -68,6 +70,10 @@ static inline int efi_get_variable_string(const char *variable, char **ret) {
         return -EOPNOTSUPP;
 }
 
+static inline int efi_get_variable_path(const char *variable, char **ret) {
+        return -EOPNOTSUPP;
+}
+
 static inline int efi_set_variable(const char *variable, const void *value, size_t size) {
         return -EOPNOTSUPP;
 }
@@ -100,3 +106,7 @@ static inline int systemd_efi_options_efivarfs_if_newer(char **line) {
         return -ENODATA;
 }
 #endif
+
+static inline char *efi_tilt_backslashes(char *s) {
+        return string_replace_char(s, '\\', '/');
+}