extmod/vfs_posix: Fix getcwd() on non-root VFS.

cwalther · cwalther · commit 0d5ead98e04b · 2023-07-31T23:23:23.000+02:00
The unwritten API contract expected of a VFS.getcwd() by mp_vfs_getcwd()
is that its return value should be either "" or "/" when the CWD is at
the root of the VFS and otherwise start with a slash and not end with a
slash. This was not correctly implemented in VfsPosix for instances with
a non-empty root - the required leading slash, if any, was cut off
because the root length includes a trailing slash. This would result in
missing slashes in the middle of the return value of os.getcwd() or in
uninitialized garbage from beyond a string's null terminator when the
CWD was at the VFS root.

Signed-off-by: Christian Walther &lt;cwalther@gmx.ch&gt;
diff --git a/extmod/vfs_posix.c b/extmod/vfs_posix.c
@@ -173,7 +173,9 @@ STATIC mp_obj_t vfs_posix_getcwd(mp_obj_t self_in) {
     if (ret == NULL) {
         mp_raise_OSError(errno);
     }
-    ret += self->root_len;
+    if (self->root_len > 0) {
+        ret += self->root_len - 1;
+    }
     return mp_obj_new_str(ret, strlen(ret));
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_1(vfs_posix_getcwd_obj, vfs_posix_getcwd);

Original file line number	Diff line number	Diff line change
`@@ -173,7 +173,9 @@ STATIC mp_obj_t vfs_posix_getcwd(mp_obj_t self_in) {`
`173`	`173`	`if (ret == NULL) {`
`174`	`174`	`mp_raise_OSError(errno);`
`175`	`175`	`}`
`176`		`- ret += self->root_len;`
	`176`	`+ if (self->root_len > 0) {`
	`177`	`+ ret += self->root_len - 1;`
	`178`	`+ }`
`177`	`179`	`return mp_obj_new_str(ret, strlen(ret));`
`178`	`180`	`}`
`179`	`181`	`STATIC MP_DEFINE_CONST_FUN_OBJ_1(vfs_posix_getcwd_obj, vfs_posix_getcwd);`