Add fchmod (#73)

* Implement & wire up fchmod

* Expose fchmod in libcos

Author: ryuukumar

kernel/include/kernel/fs/vfs.h

@@ -122,6 +122,7 @@ int do_chown (const char* path, uint64_t uid, uint64_t gid);
 int do_chroot (const char* path);
 int do_close (struct file* fd);
 int do_create (char* filename, inode** result, inode* parent);
+int do_fchmod (struct file* f, uint16_t mode);
 int do_fstat (struct file* fd, stat* buf);
 int do_fsync (struct file* fd);
 int do_getcwd (char* buf, size_t size);
@@ -149,6 +150,7 @@ uint64_t sys_chroot (uint64_t path);
 uint64_t sys_close (uint64_t fd);
 uint64_t sys_dup (uint64_t fd);
 uint64_t sys_dup2 (uint64_t oldfd, uint64_t newfd);
+uint64_t sys_fchmod (uint64_t fd, uint64_t mode);
 uint64_t sys_fstat (uint64_t fd, uint64_t buf);
 uint64_t sys_fsync (uint64_t fd);
 uint64_t sys_getcwd (uint64_t buf, uint64_t size);

kernel/include/kernel/syscall.h

@@ -52,6 +52,7 @@
 #define SYSCALL_SYS_CHOWN	 27
 #define SYSCALL_SYS_FSYNC	 28
 #define SYSCALL_SYS_CHROOT	 29
+#define SYSCALL_SYS_FCHMOD	 30
 
 /* Memory syscalls */
 

kernel/src/kernel/fs/vfs/fchmod.c

@@ -0,0 +1,35 @@
+/*
+ * fchmod.c
+ * Copyright (C) 2026  Aditya Kumar
+ *
+ * This program is free software; you can redistribute it and/or modify it under the terms of the
+ * GNU General Public License as published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
+ * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with this program; if
+ * not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include <kernel/error.h>
+#include <kernel/fs/vfs.h>
+#include <kernel/process.h>
+#include <liballoc/liballoc.h>
+
+int do_fchmod (struct file* f, uint16_t mode) {
+	if (!f) return -EINVAL;
+	inode* node = f->f_inode;
+	if (!node) return -EINVAL;
+
+	node->i_perms = (node->i_perms & ~07777) | (mode & 07777);
+	return 0;
+}
+
+uint64_t sys_fchmod (uint64_t fd, uint64_t mode) {
+	process* current = get_current_process ();
+	if (fd >= MAX_FDS || !current || !current->p_fds[fd]) return -EINVAL;
+	return (uint64_t)do_fchmod (current->p_fds[fd], mode);
+}

kernel/src/kernel/fs/vfs/vfs.c

@@ -38,6 +38,7 @@ void init_vfs (inode* absolute_root) {
 	register_syscall (SYSCALL_SYS_CLOSE, SYS1 (sys_close));
 	register_syscall (SYSCALL_SYS_DUP, SYS1 (sys_dup));
 	register_syscall (SYSCALL_SYS_DUP2, SYS2 (sys_dup2));
+	register_syscall (SYSCALL_SYS_FCHMOD, SYS2 (sys_fchmod));
 	register_syscall (SYSCALL_SYS_FSTAT, SYS2 (sys_fstat));
 	register_syscall (SYSCALL_SYS_FSYNC, SYS1 (sys_fsync));
 	register_syscall (SYSCALL_SYS_GETCWD, SYS2 (sys_getcwd));

lib/cos/include/arch/x86_64-cos/syscalls.h

@@ -50,6 +50,7 @@
 #define SYSCALL_SYS_CHOWN	 27
 #define SYSCALL_SYS_FSYNC	 28
 #define SYSCALL_SYS_CHROOT	 29
+#define SYSCALL_SYS_FCHMOD	 30
 
 /* Memory syscalls */
 

lib/cos/src/fchmod.c

@@ -0,0 +1,23 @@
+/*
+ * fchmod.c
+ * Copyright (C) 2026  Aditya Kumar
+ *
+ * This program is free software; you can redistribute it and/or modify it under the terms of the
+ * GNU General Public License as published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
+ * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with this program; if
+ * not, see <https://www.gnu.org/licenses/>.
+ */
+
+#include <arch/x86_64-cos/syscalls.h>
+#include <unistd.h>
+
+int fchmod (int __fildes, mode_t __mode) {
+	return (int)syscall_ret (
+		(long)syscall2 (SYSCALL_SYS_FCHMOD, (uint64_t)__fildes, (uint64_t)__mode));
+}