Kernel: Ignore dirfd if absolute path is given in VFS-related syscalls

To be able to do this, we add a new class called CustodyBase, which can
be resolved on-demand internally in the VirtualFileSystem resolving path
code.

When being resolved, CustodyBase will return a known custody if it was
constructed with such, if that's not the case it will provide the root
custody if the original path is absolute.
Lastly, if that's not the case as well, it will resolve the given dirfd
to provide a Custody object.

Kernel/CMakeLists.txt

@@ -135,6 +135,7 @@ set(KERNEL_SOURCES
  FileSystem/AnonymousFile.cpp
  FileSystem/BlockBasedFileSystem.cpp
  FileSystem/Custody.cpp
+ FileSystem/CustodyBase.cpp
  FileSystem/DevLoopFS/FileSystem.cpp
  FileSystem/DevLoopFS/Inode.cpp
  FileSystem/DevPtsFS/FileSystem.cpp

Kernel/FileSystem/CustodyBase.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2024, Liav A. <[email protected]>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <Kernel/FileSystem/CustodyBase.h>
+#include <Kernel/FileSystem/VirtualFileSystem.h>
+#include <Kernel/Library/KLexicalPath.h>
+#include <Kernel/Tasks/Process.h>
+
+namespace Kernel {
+
+ErrorOr<NonnullRefPtr<Custody>> CustodyBase::resolve() const
+{
+ if (m_base)
+ return *m_base;
+ if (KLexicalPath::is_absolute(m_path))
+ return VirtualFileSystem::the().root_custody();
+ return Process::current().custody_for_dirfd({}, m_dirfd);
+}
+
+}

Kernel/FileSystem/CustodyBase.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2024, Liav A. <[email protected]>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Error.h>
+#include <AK/RefPtr.h>
+#include <AK/StringView.h>
+#include <Kernel/FileSystem/Custody.h>
+
+namespace Kernel {
+
+class CustodyBase {
+public:
+ CustodyBase(int dirfd, StringView path)
+ : m_path(path)
+ , m_dirfd(dirfd)
+ {
+ }
+
+ CustodyBase(NonnullRefPtr<Custody> base)
+ : m_base(base)
+ {
+ }
+
+ CustodyBase(Custody& base)
+ : m_base(base)
+ {
+ }
+
+ CustodyBase(Custody const& base)
+ : m_base(base)
+ {
+ }
+
+ ErrorOr<NonnullRefPtr<Custody>> resolve() const;
+
+private:
+ RefPtr<Custody> const m_base;
+ StringView m_path;
+ int m_dirfd { -1 };
+};
+
+}

Kernel/FileSystem/Inode.cpp

@@ -52,7 +52,7 @@ void Inode::sync()
  }
 }
 
-ErrorOr<NonnullRefPtr<Custody>> Inode::resolve_as_link(Credentials const& credentials, Custody& base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level) const
+ErrorOr<NonnullRefPtr<Custody>> Inode::resolve_as_link(Credentials const& credentials, CustodyBase const& base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level) const
 {
  // The default implementation simply treats the stored
  // contents as a path and resolves that. That is, it

Kernel/FileSystem/Inode.h

@@ -11,6 +11,7 @@
 #include <AK/Function.h>
 #include <AK/HashTable.h>
 #include <AK/IntrusiveList.h>
+#include <Kernel/FileSystem/CustodyBase.h>
 #include <Kernel/FileSystem/FIFO.h>
 #include <Kernel/FileSystem/FileSystem.h>
 #include <Kernel/FileSystem/InodeIdentifier.h>
@@ -72,7 +73,7 @@ class Inode : public ListedRefCounted<Inode, LockType::Spinlock>
  virtual ErrorOr<void> chmod(mode_t) = 0;
  virtual ErrorOr<void> chown(UserID, GroupID) = 0;
 
- ErrorOr<NonnullRefPtr<Custody>> resolve_as_link(Credentials const&, Custody& base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level) const;
+ ErrorOr<NonnullRefPtr<Custody>> resolve_as_link(Credentials const&, CustodyBase const& base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level) const;
 
  virtual ErrorOr<int> get_block_address(int) { return ENOTSUP; }
 

Kernel/FileSystem/VirtualFileSystem.cpp

@@ -418,7 +418,7 @@ ErrorOr<void> VirtualFileSystem::traverse_directory_inode(Inode& dir_inode, Func
  });
 }
 
-ErrorOr<void> VirtualFileSystem::utime(Credentials const& credentials, StringView path, Custody& base, time_t atime, time_t mtime)
+ErrorOr<void> VirtualFileSystem::utime(Credentials const& credentials, StringView path, CustodyBase const& base, time_t atime, time_t mtime)
 {
  auto custody = TRY(resolve_path(credentials, path, base));
  auto& inode = custody->inode();
@@ -431,7 +431,7 @@ ErrorOr<void> VirtualFileSystem::utime(Credentials const& credentials, StringVie
  return {};
 }
 
-ErrorOr<void> VirtualFileSystem::utimensat(Credentials const& credentials, StringView path, Custody& base, timespec const& atime, timespec const& mtime, int options)
+ErrorOr<void> VirtualFileSystem::utimensat(Credentials const& credentials, StringView path, CustodyBase const& base, timespec const& atime, timespec const& mtime, int options)
 {
  auto custody = TRY(resolve_path(credentials, path, base, nullptr, options));
  return do_utimens(credentials, custody, atime, mtime);
@@ -454,18 +454,18 @@ ErrorOr<void> VirtualFileSystem::do_utimens(Credentials const& credentials, Cust
  return {};
 }
 
-ErrorOr<InodeMetadata> VirtualFileSystem::lookup_metadata(Credentials const& credentials, StringView path, Custody& base, int options)
+ErrorOr<InodeMetadata> VirtualFileSystem::lookup_metadata(Credentials const& credentials, StringView path, CustodyBase const& base, int options)
 {
  auto custody = TRY(resolve_path(credentials, path, base, nullptr, options));
  return custody->inode().metadata();
 }
 
-ErrorOr<NonnullRefPtr<OpenFileDescription>> VirtualFileSystem::open(Credentials const& credentials, StringView path, int options, mode_t mode, Custody& base, Optional<UidAndGid> owner)
+ErrorOr<NonnullRefPtr<OpenFileDescription>> VirtualFileSystem::open(Credentials const& credentials, StringView path, int options, mode_t mode, CustodyBase const& base, Optional<UidAndGid> owner)
 {
  return open(Process::current(), credentials, path, options, mode, base, owner);
 }
 
-ErrorOr<NonnullRefPtr<OpenFileDescription>> VirtualFileSystem::open(Process const& process, Credentials const& credentials, StringView path, int options, mode_t mode, Custody& base, Optional<UidAndGid> owner)
+ErrorOr<NonnullRefPtr<OpenFileDescription>> VirtualFileSystem::open(Process const& process, Credentials const& credentials, StringView path, int options, mode_t mode, CustodyBase const& base, Optional<UidAndGid> owner)
 {
  if ((options & O_CREAT) && (options & O_DIRECTORY))
  return EINVAL;
@@ -555,7 +555,7 @@ ErrorOr<NonnullRefPtr<OpenFileDescription>> VirtualFileSystem::open(Process cons
  return description;
 }
 
-ErrorOr<void> VirtualFileSystem::mknod(Credentials const& credentials, StringView path, mode_t mode, dev_t dev, Custody& base)
+ErrorOr<void> VirtualFileSystem::mknod(Credentials const& credentials, StringView path, mode_t mode, dev_t dev, CustodyBase const& base)
 {
  if (!is_regular_file(mode) && !is_block_device(mode) && !is_character_device(mode) && !is_fifo(mode) && !is_socket(mode))
  return EINVAL;
@@ -618,7 +618,7 @@ ErrorOr<NonnullRefPtr<OpenFileDescription>> VirtualFileSystem::create(Process co
  return description;
 }
 
-ErrorOr<void> VirtualFileSystem::mkdir(Credentials const& credentials, StringView path, mode_t mode, Custody& base)
+ErrorOr<void> VirtualFileSystem::mkdir(Credentials const& credentials, StringView path, mode_t mode, CustodyBase const& base)
 {
  // Unlike in basically every other case, where it's only the last
  // path component (the one being created) that is allowed not to
@@ -633,7 +633,8 @@ ErrorOr<void> VirtualFileSystem::mkdir(Credentials const& credentials, StringVie
  RefPtr<Custody> parent_custody;
  // FIXME: The errors returned by resolve_path_without_veil can leak information about paths that are not unveiled,
  // e.g. when the error is EACCESS or similar.
- auto result = resolve_path_without_veil(credentials, path, base, &parent_custody);
+ auto base_custody = TRY(base.resolve());
+ auto result = resolve_path_without_veil(credentials, path, base_custody, &parent_custody);
  if (!result.is_error())
  return EEXIST;
  else if (!parent_custody)
@@ -654,7 +655,7 @@ ErrorOr<void> VirtualFileSystem::mkdir(Credentials const& credentials, StringVie
  return {};
 }
 
-ErrorOr<void> VirtualFileSystem::access(Credentials const& credentials, StringView path, int mode, Custody& base, AccessFlags access_flags)
+ErrorOr<void> VirtualFileSystem::access(Credentials const& credentials, StringView path, int mode, CustodyBase const& base, AccessFlags access_flags)
 {
  auto should_follow_symlinks = !has_flag(access_flags, AccessFlags::DoNotFollowSymlinks);
  auto custody = TRY(resolve_path(credentials, path, base, nullptr, should_follow_symlinks ? 0 : O__NOERROR));
@@ -679,7 +680,7 @@ ErrorOr<void> VirtualFileSystem::access(Credentials const& credentials, StringVi
  return {};
 }
 
-ErrorOr<NonnullRefPtr<Custody>> VirtualFileSystem::open_directory(Credentials const& credentials, StringView path, Custody& base)
+ErrorOr<NonnullRefPtr<Custody>> VirtualFileSystem::open_directory(Credentials const& credentials, StringView path, CustodyBase const& base)
 {
  auto custody = TRY(resolve_path(credentials, path, base));
  auto& inode = custody->inode();
@@ -704,13 +705,13 @@ ErrorOr<void> VirtualFileSystem::chmod(Credentials const& credentials, Custody&
  return inode.chmod(mode);
 }
 
-ErrorOr<void> VirtualFileSystem::chmod(Credentials const& credentials, StringView path, mode_t mode, Custody& base, int options)
+ErrorOr<void> VirtualFileSystem::chmod(Credentials const& credentials, StringView path, mode_t mode, CustodyBase const& base, int options)
 {
  auto custody = TRY(resolve_path(credentials, path, base, nullptr, options));
  return chmod(credentials, custody, mode);
 }
 
-ErrorOr<void> VirtualFileSystem::rename(Credentials const& credentials, Custody& old_base, StringView old_path, Custody& new_base, StringView new_path)
+ErrorOr<void> VirtualFileSystem::rename(Credentials const& credentials, CustodyBase const& old_base, StringView old_path, CustodyBase const& new_base, StringView new_path)
 {
  RefPtr<Custody> old_parent_custody;
  auto old_custody = TRY(resolve_path(credentials, old_path, old_base, &old_parent_custody, O__NOERROR));
@@ -844,7 +845,7 @@ ErrorOr<void> VirtualFileSystem::chown(Credentials const& credentials, Custody&
  return inode.chown(new_uid, new_gid);
 }
 
-ErrorOr<void> VirtualFileSystem::chown(Credentials const& credentials, StringView path, UserID a_uid, GroupID a_gid, Custody& base, int options)
+ErrorOr<void> VirtualFileSystem::chown(Credentials const& credentials, StringView path, UserID a_uid, GroupID a_gid, CustodyBase const& base, int options)
 {
  auto custody = TRY(resolve_path(credentials, path, base, nullptr, options));
  return chown(credentials, custody, a_uid, a_gid);
@@ -867,7 +868,7 @@ static bool hard_link_allowed(Credentials const& credentials, Inode const& inode
  return false;
 }
 
-ErrorOr<void> VirtualFileSystem::link(Credentials const& credentials, StringView old_path, StringView new_path, Custody& base)
+ErrorOr<void> VirtualFileSystem::link(Credentials const& credentials, StringView old_path, StringView new_path, CustodyBase const& base)
 {
  // NOTE: To prevent unveil bypass by creating an hardlink after unveiling a path as read-only,
  // check that if write permission is allowed by the veil info on the old_path.
@@ -902,7 +903,7 @@ ErrorOr<void> VirtualFileSystem::link(Credentials const& credentials, StringView
  return parent_inode.add_child(old_inode, KLexicalPath::basename(new_path), old_inode.mode());
 }
 
-ErrorOr<void> VirtualFileSystem::unlink(Credentials const& credentials, StringView path, Custody& base)
+ErrorOr<void> VirtualFileSystem::unlink(Credentials const& credentials, StringView path, CustodyBase const& base)
 {
  RefPtr<Custody> parent_custody;
  auto custody = TRY(resolve_path(credentials, path, base, &parent_custody, O_WRONLY | O__NOERROR | O_UNLINK_INTERNAL));
@@ -931,10 +932,11 @@ ErrorOr<void> VirtualFileSystem::unlink(Credentials const& credentials, StringVi
  return parent_inode.remove_child(KLexicalPath::basename(path));
 }
 
-ErrorOr<void> VirtualFileSystem::symlink(Credentials const& credentials, StringView target, StringView linkpath, Custody& base)
+ErrorOr<void> VirtualFileSystem::symlink(Credentials const& credentials, StringView target, StringView linkpath, CustodyBase const& base)
 {
+ auto base_custody = TRY(base.resolve());
  // NOTE: Check that the actual target (if it exists right now) is unveiled and prevent creating symlinks on veiled paths!
- if (auto target_custody_or_error = resolve_path_without_veil(credentials, target, base, nullptr, O_RDWR, 0); !target_custody_or_error.is_error()) {
+ if (auto target_custody_or_error = resolve_path_without_veil(credentials, target, base_custody, nullptr, O_RDWR, 0); !target_custody_or_error.is_error()) {
  auto target_custody = target_custody_or_error.release_value();
  TRY(validate_path_against_process_veil(*target_custody, O_RDWR));
  }
@@ -971,7 +973,7 @@ ErrorOr<void> VirtualFileSystem::symlink(Credentials const& credentials, StringV
 }
 
 // https://pubs.opengroup.org/onlinepubs/9699919799/functions/rmdir.html
-ErrorOr<void> VirtualFileSystem::rmdir(Credentials const& credentials, StringView path, Custody& base)
+ErrorOr<void> VirtualFileSystem::rmdir(Credentials const& credentials, StringView path, CustodyBase const& base)
 {
  RefPtr<Custody> parent_custody;
  auto custody = TRY(resolve_path(credentials, path, base, &parent_custody, O_CREAT));
@@ -1151,16 +1153,17 @@ ErrorOr<void> VirtualFileSystem::validate_path_against_process_veil(StringView p
  return validate_path_against_process_veil(Process::current(), path, options);
 }
 
-ErrorOr<NonnullRefPtr<Custody>> VirtualFileSystem::resolve_path(Credentials const& credentials, StringView path, NonnullRefPtr<Custody> base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level)
+ErrorOr<NonnullRefPtr<Custody>> VirtualFileSystem::resolve_path(Credentials const& credentials, StringView path, CustodyBase const& base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level)
 {
  return resolve_path(Process::current(), credentials, path, base, out_parent, options, symlink_recursion_level);
 }
 
-ErrorOr<NonnullRefPtr<Custody>> VirtualFileSystem::resolve_path(Process const& process, Credentials const& credentials, StringView path, NonnullRefPtr<Custody> base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level)
+ErrorOr<NonnullRefPtr<Custody>> VirtualFileSystem::resolve_path(Process const& process, Credentials const& credentials, StringView path, CustodyBase const& base, RefPtr<Custody>* out_parent, int options, int symlink_recursion_level)
 {
+ auto base_custody = TRY(base.resolve());
  // FIXME: The errors returned by resolve_path_without_veil can leak information about paths that are not unveiled,
  // e.g. when the error is EACCESS or similar.
- auto custody = TRY(resolve_path_without_veil(credentials, path, base, out_parent, options, symlink_recursion_level));
+ auto custody = TRY(resolve_path_without_veil(credentials, path, base_custody, out_parent, options, symlink_recursion_level));
  if (auto result = validate_path_against_process_veil(process, *custody, options); result.is_error()) {
  if (out_parent)
  out_parent->clear();