From 4aa466b9488c94511d4b65f725cd7356af0f8743 Mon Sep 17 00:00:00 2001
From: Bananymous <oskari.alaranta@bananymous.com>
Date: Tue, 17 Sep 2024 17:16:46 +0300
Subject: [PATCH] Kernel/LibC: Implement all chmod family functions using
 fchmodat

---
 kernel/include/kernel/Process.h               |  3 +-
 kernel/kernel/Process.cpp                     | 41 +++++++++++--------
 .../libraries/LibC/include/sys/syscall.h      |  3 +-
 userspace/libraries/LibC/sys/stat.cpp         |  9 +++-
 4 files changed, 34 insertions(+), 22 deletions(-)

diff --git a/kernel/include/kernel/Process.h b/kernel/include/kernel/Process.h
index f7c98169..44d57541 100644
--- a/kernel/include/kernel/Process.h
+++ b/kernel/include/kernel/Process.h
@@ -113,8 +113,7 @@ namespace Kernel
 
 		BAN::ErrorOr<long> sys_pread(int fd, void* buffer, size_t count, off_t offset);
 
-		BAN::ErrorOr<long> sys_chmod(const char* path, mode_t mode);
-		BAN::ErrorOr<long> sys_fchmod(int fildes, mode_t mode);
+		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_socket(int domain, int type, int protocol);
diff --git a/kernel/kernel/Process.cpp b/kernel/kernel/Process.cpp
index 784cbbf1..8fba490f 100644
--- a/kernel/kernel/Process.cpp
+++ b/kernel/kernel/Process.cpp
@@ -1026,28 +1026,37 @@ namespace Kernel
 		return TRY(inode->read(offset, { (uint8_t*)buffer, count }));
 	}
 
-	BAN::ErrorOr<long> Process::sys_chmod(const char* path, mode_t mode)
+	BAN::ErrorOr<long> Process::sys_fchmodat(int fd, const char* path, mode_t mode, int flag)
 	{
 		if (mode & S_IFMASK)
 			return BAN::Error::from_errno(EINVAL);
-
-		LockGuard _(m_process_lock);
-		TRY(validate_string_access(path));
-
-		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->chmod(mode));
-
-		return 0;
-	}
-
-	BAN::ErrorOr<long> Process::sys_fchmod(int fildes, mode_t mode)
-	{
-		if (mode & S_IFMASK)
+		if (flag & ~AT_SYMLINK_NOFOLLOW)
 			return BAN::Error::from_errno(EINVAL);
+		if (flag == AT_SYMLINK_NOFOLLOW)
+			flag = O_NOFOLLOW;
 
 		LockGuard _(m_process_lock);
-		auto inode = TRY(m_open_file_descriptors.inode_of(fildes));
+		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 (!m_credentials.is_superuser() && inode->uid() != m_credentials.euid())
+		{
+			dwarnln("cannot chmod uid {} vs {}", inode->uid(), m_credentials.euid());
+			return BAN::Error::from_errno(EPERM);
+		}
+
 		TRY(inode->chmod(mode));
 
 		return 0;
diff --git a/userspace/libraries/LibC/include/sys/syscall.h b/userspace/libraries/LibC/include/sys/syscall.h
index 071c70c7..5aef859c 100644
--- a/userspace/libraries/LibC/include/sys/syscall.h
+++ b/userspace/libraries/LibC/include/sys/syscall.h
@@ -51,8 +51,7 @@ __BEGIN_DECLS
 	O(SYS_MUNMAP,			munmap) 		\
 	O(SYS_TTY_CTRL,			tty_ctrl)		\
 	O(SYS_POWEROFF,			poweroff)		\
-	O(SYS_CHMOD,			chmod)			\
-	O(SYS_FCHMOD,			fchmod)			\
+	O(SYS_FCHMODAT,			fchmodat)		\
 	O(SYS_CREATE_DIR,		create_dir)		\
 	O(SYS_UNLINK,			unlink) 		\
 	O(SYS_READLINKAT,		readlinkat)		\
diff --git a/userspace/libraries/LibC/sys/stat.cpp b/userspace/libraries/LibC/sys/stat.cpp
index ad3189e9..d1ffb259 100644
--- a/userspace/libraries/LibC/sys/stat.cpp
+++ b/userspace/libraries/LibC/sys/stat.cpp
@@ -9,12 +9,17 @@ mode_t __umask = 0;
 
 int chmod(const char* path, mode_t mode)
 {
-	return syscall(SYS_CHMOD, path, mode);
+	return fchmodat(AT_FDCWD, path, mode, 0);
 }
 
 int fchmod(int fildes, mode_t mode)
 {
-	return syscall(SYS_FCHMOD, fildes, mode);
+	return fchmodat(fildes, nullptr, mode, 0);
+}
+
+int fchmodat(int fildes, const char* path, mode_t mode, int flag)
+{
+	return syscall(SYS_FCHMODAT, fildes, path, mode, flag);
 }
 
 int fstat(int fildes, struct stat* buf)