libc: Prefer to implement memfd on top of shm

since shm can work in protected and kernel mode too

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

libs/libc/misc/Kconfig

@@ -81,9 +81,25 @@ config LIBC_FTOK_VFS_PATH
 	---help---
 		The relative path to where ftok will exist in the root namespace.
 
+choice
+	prompt "Select memfd implementation"
+
+config LIBC_MEMFD_SHMFS
+	bool "memfd base on shmfs"
+	depends on FS_SHMFS
+
+config LIBC_MEMFD_TMPFS
+	bool "memfd base on tmpfs"
+	depends on FS_TMPFS
+
+config LIBC_MEMFD_ERROR
+	bool "memfd return error"
+
+endchoice
+
 config LIBC_MEM_FD_VFS_PATH
 	string "Relative path to memfd storage"
 	default "memfd"
-	depends on FS_TMPFS
+	depends on !LIBC_MEMFD_ERROR
 	---help---
 		The relative path to where memfd will exist in the tmpfs namespace.

libs/libc/misc/lib_memfd.c

@@ -32,8 +32,13 @@
  * Pre-processor Definitions
  ****************************************************************************/
 
-#define LIBC_MEM_FD_VFS_PATH \
+#ifdef CONFIG_LIBC_MEMFD_TMPFS
-          CONFIG_LIBC_TMPDIR "/" CONFIG_LIBC_MEM_FD_VFS_PATH "/%s"
+#  define LIBC_MEM_FD_VFS_PATH CONFIG_LIBC_TMPDIR "/" CONFIG_LIBC_MEM_FD_VFS_PATH
+#else
+#  define LIBC_MEM_FD_VFS_PATH CONFIG_LIBC_MEM_FD_VFS_PATH
+#endif
+
+#define LIBC_MEM_FD_VFS_PATH_FMT LIBC_MEM_FD_VFS_PATH "/%s"
 
 /****************************************************************************
  * Public Functions
@@ -41,13 +46,17 @@
 
 int memfd_create(FAR const char *name, unsigned int flags)
 {
-#ifdef CONFIG_FS_TMPFS
+#ifdef CONFIG_LIBC_MEMFD_ERROR
-  char path[PATH_MAX];
-
-  snprintf(path, sizeof(path), LIBC_MEM_FD_VFS_PATH, name);
-  return open(path, O_RDWR | flags);
-#else
   set_errno(ENOSYS);
   return -1;
+#else
+  char path[PATH_MAX];
+
+  snprintf(path, sizeof(path), LIBC_MEM_FD_VFS_PATH_FMT, name);
+#  ifdef CONFIG_LIBC_MEMFD_SHMFS
+  return shm_open(path, O_RDWR | flags, 0660);
+#  else
+  return open(path, O_RDWR | flags, 0660);
+#  endif
 #endif
 }