LibCore: Add wrapper for fstatat()

`Core::System::fstatat()` is similar to our standard `Core::System`
wrappers.

`Core::Directory::stat(relative_path, flags)` is a convenience method if
you already have a Directory, to stat a file relative to it.

Userland/Libraries/LibCore/Directory.cpp

@@ -88,6 +88,11 @@ ErrorOr<NonnullOwnPtr<File>> Directory::open(StringView filename, File::OpenMode
     return File::adopt_fd(fd, mode);
 }
 
+ErrorOr<struct stat> Directory::stat(StringView filename, int flags) const
+{
+    return System::fstatat(m_directory_fd, filename, flags);
+}
+
 ErrorOr<struct stat> Directory::stat() const
 {
     return System::fstat(m_directory_fd);

Userland/Libraries/LibCore/Directory.h

@@ -41,6 +41,7 @@ public:
     static ErrorOr<Directory> adopt_fd(int fd, LexicalPath path);
 
     ErrorOr<NonnullOwnPtr<File>> open(StringView filename, File::OpenMode mode) const;
+    ErrorOr<struct stat> stat(StringView filename, int flags) const;
     ErrorOr<struct stat> stat() const;
     int fd() const { return m_directory_fd; }
 

Userland/Libraries/LibCore/System.cpp

@@ -1,7 +1,7 @@
 /*
  * Copyright (c) 2021-2022, Andreas Kling <kling@serenityos.org>
  * Copyright (c) 2021-2022, Kenneth Myhra <kennethmyhra@serenityos.org>
- * Copyright (c) 2021-2022, Sam Atkins <atkinssj@serenityos.org>
+ * Copyright (c) 2021-2023, Sam Atkins <atkinssj@serenityos.org>
  * Copyright (c) 2022, Matthias Zimmerman <matthias291999@gmail.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
@@ -380,6 +380,22 @@ ErrorOr<struct stat> fstat(int fd)
     return st;
 }
 
+ErrorOr<struct stat> fstatat(int fd, StringView path, int flags)
+{
+    if (!path.characters_without_null_termination())
+        return Error::from_syscall("fstatat"sv, -EFAULT);
+
+    struct stat st = {};
+#ifdef AK_OS_SERENITY
+    Syscall::SC_stat_params params { { path.characters_without_null_termination(), path.length() }, &st, fd, !(flags & AT_SYMLINK_NOFOLLOW) };
+    int rc = syscall(SC_stat, &params);
+#else
+    DeprecatedString path_string = path;
+    int rc = ::fstatat(fd, path_string.characters(), &st, flags);
+#endif
+    HANDLE_SYSCALL_RETURN_VALUE("fstatat", rc, st);
+}
+
 ErrorOr<int> fcntl(int fd, int command, ...)
 {
     va_list ap;

Userland/Libraries/LibCore/System.h

@@ -116,6 +116,7 @@ ErrorOr<sig_t> signal(int signal, sig_t handler);
 ErrorOr<sighandler_t> signal(int signal, sighandler_t handler);
 #endif
 ErrorOr<struct stat> fstat(int fd);
+ErrorOr<struct stat> fstatat(int fd, StringView path, int flags);
 ErrorOr<int> fcntl(int fd, int command, ...);
 ErrorOr<void*> mmap(void* address, size_t, int protection, int flags, int fd, off_t, size_t alignment = 0, StringView name = {});
 ErrorOr<void> munmap(void* address, size_t);