Kernel: Implement recursion limit on path resolution

Cautiously use 5 as a limit for now so that we don't blow the stack.
This can be increased in the future if we are sure that we won't be
blowing the stack, or if the implementation is changed to not use
recursion :^)

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -11,6 +11,7 @@
 //#define VFS_DEBUG
 
 static VFS* s_the;
+static constexpr int symlink_recursion_limit { 5 }; // FIXME: increase?
 
 VFS& VFS::the()
 {
@@ -615,7 +616,6 @@ InodeIdentifier VFS::Mount::host() const
     return m_host_custody->inode().identifier();
 }
 
-
 void VFS::for_each_mount(Function<void(const Mount&)> callback) const
 {
     for (auto& mount : m_mounts) {
@@ -635,10 +635,12 @@ Custody& VFS::root_custody()
     return *m_root_custody;
 }
 
-KResultOr<NonnullRefPtr<Custody>> VFS::resolve_path(StringView path, Custody& base, RefPtr<Custody>* parent_custody, int options)
+KResultOr<NonnullRefPtr<Custody>> VFS::resolve_path(StringView path, Custody& base, RefPtr<Custody>* parent_custody, int options, int symlink_recursion_level)
 {
     // FIXME: resolve_path currently doesn't deal with .. and . . If path is ../. and base is /home/anon, it returns
     //        /home/anon/../. instead of /home .
+    if (symlink_recursion_level >= symlink_recursion_limit)
+        return KResult(-ELOOP);
 
     if (path.is_empty())
         return KResult(-EINVAL);
@@ -712,13 +714,8 @@ KResultOr<NonnullRefPtr<Custody>> VFS::resolve_path(StringView path, Custody& ba
             if (!symlink_contents)
                 return KResult(-ENOENT);
 
-            // FIXME: We should limit the recursion here and return -ELOOP if it goes to deep.
-            auto symlink_target = resolve_path(
-                StringView(symlink_contents.data(),
-                    symlink_contents.size()),
-                current_parent,
-                parent_custody,
-                options);
+            auto symlink_path = StringView(symlink_contents.data(), symlink_contents.size());
+            auto symlink_target = resolve_path(symlink_path, current_parent, parent_custody, options, symlink_recursion_level + 1);
 
             if (symlink_target.is_error())
                 return symlink_target;
@@ -731,7 +728,7 @@ KResultOr<NonnullRefPtr<Custody>> VFS::resolve_path(StringView path, Custody& ba
                 return symlink_target;
 
             StringView remaining_path = path.substring_view_starting_from_substring(parts[i + 1]);
-            return resolve_path(remaining_path, *symlink_target.value(), parent_custody, options);
+            return resolve_path(remaining_path, *symlink_target.value(), parent_custody, options, symlink_recursion_level + 1);
         }
     }
     return custody_chain.last();

Kernel/FileSystem/VirtualFileSystem.h

@@ -88,7 +88,7 @@ public:
     void sync();
 
     Custody& root_custody();
-    KResultOr<NonnullRefPtr<Custody>> resolve_path(StringView path, Custody& base, RefPtr<Custody>* parent = nullptr, int options = 0);
+    KResultOr<NonnullRefPtr<Custody>> resolve_path(StringView path, Custody& base, RefPtr<Custody>* parent = nullptr, int options = 0, int symlink_recursion_level = 0);
 
 private:
     friend class FileDescription;