Kernel: Fix awkward bug where "touch /foo/bar/baz" could create "/baz"

To accomodate file creation, path resolution optionally returns the
last valid parent directory seen while traversing the path.

Clients will then interpret "ENOENT, but I have a parent for you" as
meaning that the file doesn't exist, but its immediate parent directory
does. The client then goes ahead and creates a new file.

In the case of "/foo/bar/baz" where there is no "/foo", it would fail
with ENOENT and "/" as the last seen parent directory, causing e.g the
open() syscall to create "/baz".

Covered by test_io.

1 files changed, 12 insertions, 1 deletions

diff --git a/Kernel/FileSystem/VirtualFileSystem.cpp b/Kernel/FileSystem/VirtualFileSystem.cpp
index 64317dec64..5539032e69 100644
--- a/Kernel/FileSystem/VirtualFileSystem.cpp
+++ b/Kernel/FileSystem/VirtualFileSystem.cpp
@@ -692,8 +692,19 @@ KResultOr<NonnullRefPtr<Custody>> VFS::resolve_path(StringView path, Custody& ba
 
         auto& current_parent = custody_chain.last();
         crumb_id = crumb_inode->lookup(part);
-        if (!crumb_id.is_valid())
+        if (!crumb_id.is_valid()) {
+            if (i != parts.size() - 1) {
+                // We didn't find the filename we were looking for,
+                // and we didn't even reach the last path part.
+                // (ENOENT with non-null parent_custody) signals to caller that
+                // we found the immediate parent of the file, but the file itself
+                // does not exist yet.
+                // Since this is not the immediate parent, clear parent_custody.
+                if (parent_custody)
+                    *parent_custody = nullptr;
+            }
             return KResult(-ENOENT);
+        }
         if (auto mount = find_mount_for_host(crumb_id))
             crumb_id = mount->guest();
         if (inode_was_root_at_head_of_loop && crumb_id.is_root_inode() && !is_vfs_root(crumb_id) && part == "..") {