[PATCH 1/2] Implement fchmodat4 syscall

[William Orr <will@worrbase.com>]

Adds fchmodat4 which more closely matches POSIX by taking 4 arguments,
including the flags argument. flags are the same as fchownat(2), implementing
both AT_SYMLINK_NOFOLLOW and AT_EMPTY_PATH

Based heavily off of Andrew Ayer's patch from 2012.
---
 fs/open.c                | 19 +++++++++++++++++--
 include/linux/syscalls.h |  2 ++
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 98e5a52..00dd0f7 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -504,6 +504,9 @@ static int chmod_common(struct path *path, umode_t mode)
 	struct iattr newattrs;
 	int error;
 
+	if (S_ISLNK(inode->i_mode))
+		return -EOPNOTSUPP;
+
 	error = mnt_want_write(path->mnt);
 	if (error)
 		return error;
@@ -541,9 +544,21 @@ SYSCALL_DEFINE2(fchmod, unsigned int, fd, umode_t, mode)
 
 SYSCALL_DEFINE3(fchmodat, int, dfd, const char __user *, filename, umode_t, mode)
 {
+	return sys_fchmodat4(dfd, filename, mode, 0);
+}
+
+SYSCALL_DEFINE4(fchmodat4, int, dfd, const char __user *, filename, umode_t, mode, int, flags)
+{
 	struct path path;
 	int error;
-	unsigned int lookup_flags = LOOKUP_FOLLOW;
+	unsigned int lookup_flags;
+
+	if (flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH))
+		return -EINVAL;
+
+	lookup_flags = (flags & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
+	if (flags & AT_EMPTY_PATH)
+		lookup_flags |= LOOKUP_EMPTY;
 retry:
 	error = user_path_at(dfd, filename, lookup_flags, &path);
 	if (!error) {
@@ -559,7 +574,7 @@ retry:
 
 SYSCALL_DEFINE2(chmod, const char __user *, filename, umode_t, mode)
 {
-	return sys_fchmodat(AT_FDCWD, filename, mode);
+	return sys_fchmodat4(AT_FDCWD, filename, mode, 0);
 }
 
 static int chown_common(struct path *path, uid_t user, gid_t group)
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 76d1e38..d6e9602 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -769,6 +769,8 @@ asmlinkage long sys_futimesat(int dfd, const char __user *filename,
 asmlinkage long sys_faccessat(int dfd, const char __user *filename, int mode);
 asmlinkage long sys_fchmodat(int dfd, const char __user * filename,
 			     umode_t mode);
+asmlinkage long sys_fchmodat4(int dfd, const char __user * filename,
+			     umode_t mode, int flags);
 asmlinkage long sys_fchownat(int dfd, const char __user *filename, uid_t user,
 			     gid_t group, int flag);
 asmlinkage long sys_openat(int dfd, const char __user *filename, int flags,
-- 
2.1.0