From e431e90b20d60d15816de81aef9adb67e0e939c3 Mon Sep 17 00:00:00 2001
From: Bananymous <oskari.alaranta@bananymous.com>
Date: Tue, 17 Sep 2024 18:35:01 +0300
Subject: [PATCH] Kernel/LibC: Implement all chown family function with
 fchownat

---
 kernel/include/kernel/Process.h               |  2 +-
 kernel/kernel/Process.cpp                     | 37 ++++++++++++++++---
 .../libraries/LibC/include/sys/syscall.h      |  2 +-
 userspace/libraries/LibC/unistd.cpp           | 17 ++++++++-
 4 files changed, 49 insertions(+), 9 deletions(-)

diff --git a/kernel/include/kernel/Process.h b/kernel/include/kernel/Process.h
index 44d575416e..ac1b7f11cb 100644
--- a/kernel/include/kernel/Process.h
+++ b/kernel/include/kernel/Process.h
@@ -114,7 +114,7 @@ namespace Kernel
 		BAN::ErrorOr<long> sys_pread(int fd, void* buffer, size_t count, off_t offset);
 
 		BAN::ErrorOr<long> sys_fchmodat(int fd, const char* path, mode_t mode, int flag);
-		BAN::ErrorOr<long> sys_chown(const char* path, uid_t uid, gid_t gid);
+		BAN::ErrorOr<long> sys_fchownat(int fd, const char* path, uid_t uid, gid_t gid, int flag);
 
 		BAN::ErrorOr<long> sys_socket(int domain, int type, int protocol);
 		BAN::ErrorOr<long> sys_getsockname(int socket, sockaddr* address, socklen_t* address_len);
diff --git a/kernel/kernel/Process.cpp b/kernel/kernel/Process.cpp
index 8fba490f58..7dd25f0cf3 100644
--- a/kernel/kernel/Process.cpp
+++ b/kernel/kernel/Process.cpp
@@ -1062,14 +1062,39 @@ namespace Kernel
 		return 0;
 	}
 
-	BAN::ErrorOr<long> Process::sys_chown(const char* path, uid_t uid, gid_t gid)
+	BAN::ErrorOr<long> Process::sys_fchownat(int fd, const char* path, uid_t uid, gid_t gid, int flag)
 	{
-		LockGuard _(m_process_lock);
-		TRY(validate_string_access(path));
+		if (flag & ~AT_SYMLINK_NOFOLLOW)
+			return BAN::Error::from_errno(EINVAL);
+		if (flag == AT_SYMLINK_NOFOLLOW)
+			flag = O_NOFOLLOW;
 
-		auto absolute_path = TRY(absolute_path_of(path));
-		auto file = TRY(VirtualFileSystem::get().file_from_absolute_path(m_credentials, absolute_path, O_WRONLY));
-		TRY(file.inode->chown(uid, gid));
+		LockGuard _(m_process_lock);
+		if (path)
+			TRY(validate_string_access(path));
+
+		VirtualFileSystem::File parent_file;
+		if (path && path[0] == '/')
+			parent_file = VirtualFileSystem::get().root_file();
+		else if (fd == AT_FDCWD)
+			parent_file = TRY(m_working_directory.clone());
+		else
+			parent_file = TRY(m_open_file_descriptors.file_of(fd));
+
+		auto inode = path
+			? TRY(VirtualFileSystem::get().file_from_relative_path(parent_file, m_credentials, path, flag)).inode
+			: parent_file.inode;
+
+		if (uid != -1 && !m_credentials.is_superuser())
+			return BAN::Error::from_errno(EPERM);
+		if (gid != -1 && !m_credentials.is_superuser() && (m_credentials.euid() != uid || !m_credentials.has_egid(gid)))
+			return BAN::Error::from_errno(EPERM);
+
+		if (uid == -1)
+			uid = inode->uid();
+		if (gid == -1)
+			gid = inode->gid();
+		TRY(inode->chown(uid, gid));
 
 		return 0;
 	}
diff --git a/userspace/libraries/LibC/include/sys/syscall.h b/userspace/libraries/LibC/include/sys/syscall.h
index 5aef859c7f..bc5d9b27f5 100644
--- a/userspace/libraries/LibC/include/sys/syscall.h
+++ b/userspace/libraries/LibC/include/sys/syscall.h
@@ -57,7 +57,7 @@ __BEGIN_DECLS
 	O(SYS_READLINKAT,		readlinkat)		\
 	O(SYS_MSYNC,			msync)			\
 	O(SYS_PREAD,			pread)			\
-	O(SYS_CHOWN,			chown)			\
+	O(SYS_FCHOWNAT,			fchownat)		\
 	O(SYS_LOAD_KEYMAP,		load_keymap)	\
 	O(SYS_SOCKET,			socket)			\
 	O(SYS_BIND,				bind)			\
diff --git a/userspace/libraries/LibC/unistd.cpp b/userspace/libraries/LibC/unistd.cpp
index 62493da1d8..5553373145 100644
--- a/userspace/libraries/LibC/unistd.cpp
+++ b/userspace/libraries/LibC/unistd.cpp
@@ -315,7 +315,22 @@ int chdir(const char* path)
 
 int chown(const char* path, uid_t owner, gid_t group)
 {
-	return syscall(SYS_CHOWN, path, owner, group);
+	return fchownat(AT_FDCWD, path, owner, group, 0);
+}
+
+int lchown(const char* path, uid_t owner, gid_t group)
+{
+	return fchownat(AT_FDCWD, path, owner, group, AT_SYMLINK_NOFOLLOW);
+}
+
+int fchown(int fildes, uid_t owner, gid_t group)
+{
+	return fchownat(fildes, nullptr, owner, group, 0);
+}
+
+int fchownat(int fd, const char* path, uid_t owner, gid_t group, int flag)
+{
+	return syscall(SYS_FCHOWNAT, fd, path, owner, group, flag);
 }
 
 void sync(void)