libsubid: Add routine to free allocated memory

lib/nss.c

@@ -131,6 +131,11 @@ void nss_init(const char *nsswitch_path) {
 		fprintf(shadow_logfd, "%s did not provide @find_subid_owners@\n", libname);
 		goto close_lib;
 	}
+	subid_nss->free= dlsym(h, "shadow_subid_free");
+	if (!subid_nss->free) {
+		// Fallback to default `free()` for backward compatibility.
+		subid_nss->free = free;
+	}
 	subid_nss->handle = h;
 	goto done;
 

lib/prototypes.h

@@ -284,6 +284,19 @@ struct subid_nss_ops {
 	 */
 	enum subid_status (*find_subid_owners)(unsigned long id, enum subid_type id_type, uid_t **uids, int *count);
 
+	/*
+	 * nss_free: free a memory block allocated by a subid plugin.
+	 *
+	 * @ptr - a pointer to a memory block to deallocate
+	 *
+	 * Some routines of subid_nss_ops allocate memory which should be freed by
+	 * caller after use. In order to deallocate that memory block, one should
+	 * use this routine to release that memory. By default, this function
+	 * pointer is set to free() for backward compatibility. However, it is
+	 * strongly recommended to define this routine explicitly.
+	 */
+	void (*free)(void *ptr);
+
 	/* The dlsym handle to close */
 	void *handle;
 };

lib/subordinateio.c

@@ -1117,6 +1117,15 @@ bool release_subid_range(struct subordinate_range *range, enum subid_type id_typ
 	return ret;
 }
 
+void free_subid_pointer(void *ptr) {
+	struct subid_nss_ops *h = get_subid_nss_handle();
+	if (h) {
+		h->free(ptr);
+	} else {
+		free(ptr);
+	}
+}
+
 #else				/* !ENABLE_SUBIDS */
 extern int ISO_C_forbids_an_empty_translation_unit;
 #endif				/* !ENABLE_SUBIDS */

lib/subordinateio.h

@@ -43,6 +43,9 @@ extern int sub_gid_unlock (void);
 extern int sub_gid_add (const char *owner, gid_t start, unsigned long count);
 extern int sub_gid_remove (const char *owner, gid_t start, unsigned long count);
 extern uid_t sub_gid_find_free_range(gid_t min, gid_t max, unsigned long count);
+
+extern void free_subid_pointer(void *ptr);
+
 #endif				/* ENABLE_SUBIDS */
 
 #endif

libsubid/api.c

@@ -42,6 +42,10 @@ bool subid_init(const char *progname, FILE * logfd)
 	return true;
 }
 
+void subid_free(void *ptr) {
+	free_subid_pointer(ptr);
+}
+
 static
 int get_subid_ranges(const char *owner, enum subid_type id_type, struct subid_range **ranges)
 {

libsubid/subid.h.in

@@ -55,6 +55,19 @@ extern "C" {
  */
 bool subid_init(const char *progname, FILE *logfd);
 
+/*
+ * subid_free: free memory allocated in any subid_* function
+ *
+ * @ptr: Pointer to a memory block to release.
+ *
+ * Some functions like @subid_get_uid_ranges allocate memory internally. As
+ * soon as a result is no longer needed, it should be freed with this routine.
+ * Initially, default function `free()` was used. Thus for backward
+ * compatibility this function falls back to `free()` if a plugin does not
+ * explicitly specify routine to free allocated memory.
+ */
+void subid_free(void *ptr);
+
 /*
  * subid_get_uid_ranges: return a list of UID ranges for a user
  *

src/getsubids.c

@@ -44,6 +44,6 @@ int main(int argc, char *argv[])
 		printf("%d: %s %lu %lu\n", i, owner,
 			ranges[i].start, ranges[i].count);
 	}
-	free(ranges);
+	subid_free(ranges);
 	return 0;
 }