vfs_posix: Add MICROPY_VFS_POSIX_READONLY.

jepler · jepler · commit cc65036335eb · 2025-07-19T10:27:49.000-05:00
When this configuration flag is set, VfsPosix instances will
always be created "read only".

This flag is when fuzzing micropython: Without VfsPosix,
the fuzzing input script cannot be read; but with writable
VfsPosix, fuzzing scripts can access and potentially damage
the host filesystem.

Signed-off-by: Jeff Epler &lt;jepler@gmail.com&gt;
diff --git a/extmod/vfs_posix.c b/extmod/vfs_posix.c
@@ -137,7 +137,7 @@ static mp_obj_t vfs_posix_make_new(const mp_obj_type_t *type, size_t n_args, siz
         vstr_add_char(&vfs->root, '/');
     }
     vfs->root_len = vfs->root.len;
-    vfs->readonly = false;
+    vfs->readonly = MICROPY_VFS_POSIX_READONLY;
 
     return MP_OBJ_FROM_PTR(vfs);
 }
@@ -161,7 +161,7 @@ static mp_obj_t vfs_posix_umount(mp_obj_t self_in) {
 static MP_DEFINE_CONST_FUN_OBJ_1(vfs_posix_umount_obj, vfs_posix_umount);
 
 static bool vfs_posix_is_readonly(mp_obj_vfs_posix_t *self) {
-    return self->readonly;
+    return MICROPY_VFS_POSIX_READONLY || self->readonly;
 }
 
 static void vfs_posix_require_writable(mp_obj_t self_in) {
diff --git a/py/mpconfig.h b/py/mpconfig.h
@@ -1084,6 +1084,11 @@ typedef time_t mp_timestamp_t;
 #define MICROPY_VFS_POSIX (0)
 #endif
 
+// When enabled, the POSIX VFS is always read-only.
+#ifndef MICROPY_VFS_POSIX_READONLY
+#define MICROPY_VFS_POSIX_READONLY (0)
+#endif
+
 // Support for VFS FAT component, to mount a FAT filesystem within VFS
 #ifndef MICROPY_VFS_FAT
 #define MICROPY_VFS_FAT (0)

Original file line number	Diff line number	Diff line change
`@@ -137,7 +137,7 @@ static mp_obj_t vfs_posix_make_new(const mp_obj_type_t *type, size_t n_args, siz`
`137`	`137`	`vstr_add_char(&vfs->root, '/');`
`138`	`138`	`}`
`139`	`139`	`vfs->root_len = vfs->root.len;`
`140`		`- vfs->readonly = false;`
	`140`	`+ vfs->readonly = MICROPY_VFS_POSIX_READONLY;`
`141`	`141`
`142`	`142`	`return MP_OBJ_FROM_PTR(vfs);`
`143`	`143`	`}`
`@@ -161,7 +161,7 @@ static mp_obj_t vfs_posix_umount(mp_obj_t self_in) {`
`161`	`161`	`static MP_DEFINE_CONST_FUN_OBJ_1(vfs_posix_umount_obj, vfs_posix_umount);`
`162`	`162`
`163`	`163`	`static bool vfs_posix_is_readonly(mp_obj_vfs_posix_t *self) {`
`164`		`- return self->readonly;`
	`164`	`+ return MICROPY_VFS_POSIX_READONLY \|\| self->readonly;`
`165`	`165`	`}`
`166`	`166`
`167`	`167`	`static void vfs_posix_require_writable(mp_obj_t self_in) {`