fs: add do_fchownat(), ksys_fchown() helpers and ksys_{,l}chown() wrappers

Using the fs-interal do_fchownat() wrapper allows us to get rid of
fs-internal calls to the sys_fchownat() syscall.

Introducing the ksys_fchown() helper and the ksys_{,}chown() wrappers
allows us to avoid the in-kernel calls to the sys_{,l,f}chown() syscalls.
The ksys_ prefix denotes that these functions are meant as a drop-in
replacement for the syscalls. In particular, they use the same calling
convention as sys_{,l,f}chown().

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

1 files changed, 17 insertions, 6 deletions

diff --git a/fs/open.c b/fs/open.c
index 0fc8188be31a..7b2eccb541f2 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -645,8 +645,8 @@ retry_deleg:
 	return error;
 }
 
-SYSCALL_DEFINE5(fchownat, int, dfd, const char __user *, filename, uid_t, user,
-		gid_t, group, int, flag)
+int do_fchownat(int dfd, const char __user *filename, uid_t user, gid_t group,
+		int flag)
 {
 	struct path path;
 	int error = -EINVAL;
@@ -677,18 +677,24 @@ out:
 	return error;
 }
 
+SYSCALL_DEFINE5(fchownat, int, dfd, const char __user *, filename, uid_t, user,
+		gid_t, group, int, flag)
+{
+	return do_fchownat(dfd, filename, user, group, flag);
+}
+
 SYSCALL_DEFINE3(chown, const char __user *, filename, uid_t, user, gid_t, group)
 {
-	return sys_fchownat(AT_FDCWD, filename, user, group, 0);
+	return do_fchownat(AT_FDCWD, filename, user, group, 0);
 }
 
 SYSCALL_DEFINE3(lchown, const char __user *, filename, uid_t, user, gid_t, group)
 {
-	return sys_fchownat(AT_FDCWD, filename, user, group,
-			    AT_SYMLINK_NOFOLLOW);
+	return do_fchownat(AT_FDCWD, filename, user, group,
+			   AT_SYMLINK_NOFOLLOW);
 }
 
-SYSCALL_DEFINE3(fchown, unsigned int, fd, uid_t, user, gid_t, group)
+int ksys_fchown(unsigned int fd, uid_t user, gid_t group)
 {
 	struct fd f = fdget(fd);
 	int error = -EBADF;
@@ -708,6 +714,11 @@ out:
 	return error;
 }
 
+SYSCALL_DEFINE3(fchown, unsigned int, fd, uid_t, user, gid_t, group)
+{
+	return ksys_fchown(fd, user, group);
+}
+
 int open_check_o_direct(struct file *f)
 {
 	/* NB: we're sure to have correct a_ops only after f_op->open */