From 87a62f4def45da6d03bd7e60d8a0090c76143753 Mon Sep 17 00:00:00 2001
From: Max Wipfli <mail@maxwipfli.ch>
Date: Tue, 6 Jul 2021 11:16:17 +0200
Subject: [PATCH] Kernel: Add KLexicalPath

This adds KLexicalPath, which are a few static functions which aim to
mostly emulate AK::LexicalPath. They are however constrained to work
with absolute paths only, containing no '.' or '..' path segments and no
consecutive slashes. This way, it is possible to avoid use StringView
for the return values and thus avoid allocating new String objects.

As explained above, the functions are currently very strict about the
allowed input paths. This seems to not be a problem currently. Since the
functions VERIFY this, potential bugs caused by this will become
immediately obvious.
---
 Kernel/CMakeLists.txt   |  1 +
 Kernel/KLexicalPath.cpp | 58 +++++++++++++++++++++++++++++++++++++++++
 Kernel/KLexicalPath.h   | 19 ++++++++++++++
 3 files changed, 78 insertions(+)
 create mode 100644 Kernel/KLexicalPath.cpp
 create mode 100644 Kernel/KLexicalPath.h

diff --git a/Kernel/CMakeLists.txt b/Kernel/CMakeLists.txt
index dbaebcb06d..ecd2c37a20 100644
--- a/Kernel/CMakeLists.txt
+++ b/Kernel/CMakeLists.txt
@@ -128,6 +128,7 @@ set(KERNEL_SOURCES
     Interrupts/SpuriousInterruptHandler.cpp
     Interrupts/UnhandledInterruptHandler.cpp
     KBufferBuilder.cpp
+    KLexicalPath.cpp
     KString.cpp
     KSyms.cpp
     Lock.cpp
diff --git a/Kernel/KLexicalPath.cpp b/Kernel/KLexicalPath.cpp
new file mode 100644
index 0000000000..8e9cc34c98
--- /dev/null
+++ b/Kernel/KLexicalPath.cpp
@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2021, Max Wipfli <max.wipfli@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/KLexicalPath.h>
+
+namespace Kernel::KLexicalPath {
+
+bool is_absolute(StringView const& path)
+{
+    return !path.is_empty() && path[0] == '/';
+}
+
+bool is_canonical(StringView const& path)
+{
+    // FIXME: This can probably be done more efficiently.
+    if (path.is_empty())
+        return false;
+    if (path.ends_with('/') && path.length() != 1)
+        return false;
+    if (path.starts_with("./"sv) || path.contains("/./"sv) || path.ends_with("/."sv))
+        return false;
+    if (path.starts_with("../"sv) || path.contains("/../"sv) || path.ends_with("/.."))
+        return false;
+    if (path.contains("//"sv))
+        return false;
+    return true;
+}
+
+StringView basename(StringView const& path)
+{
+    auto slash_index = path.find_last('/');
+    if (!slash_index.has_value()) {
+        VERIFY(!path.is_empty());
+        return path;
+    }
+    auto basename = path.substring_view(*slash_index + 1);
+    VERIFY(!basename.is_empty() && basename != "."sv && basename != ".."sv);
+    return basename;
+}
+
+StringView dirname(StringView const& path)
+{
+    VERIFY(is_canonical(path));
+    auto slash_index = path.find_last('/');
+    VERIFY(slash_index.has_value());
+    return path.substring_view(0, *slash_index);
+}
+
+Vector<StringView> parts(StringView const& path)
+{
+    VERIFY(is_canonical(path));
+    return path.split_view('/');
+}
+
+}
diff --git a/Kernel/KLexicalPath.h b/Kernel/KLexicalPath.h
new file mode 100644
index 0000000000..87b7bea87a
--- /dev/null
+++ b/Kernel/KLexicalPath.h
@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2021, Max Wipfli <max.wipfli@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/StringView.h>
+
+namespace Kernel::KLexicalPath {
+
+bool is_absolute(StringView const&);
+bool is_canonical(StringView const&);
+StringView basename(StringView const&);
+StringView dirname(StringView const&);
+Vector<StringView> parts(StringView const&);
+
+}