fs: add do_symlinkat() helper and ksys_symlink() wrapper; remove in-kernel calls to syscall

Using the fs-internal do_symlinkat() helper allows us to get rid of
fs-internal calls to the sys_symlinkat() syscall.

Introducing the ksys_symlink() wrapper allows us to avoid the in-kernel
calls to the sys_symlink() syscall. The ksys_ prefix denotes that this
function is meant as a drop-in replacement for the syscall. In particular,
it uses the same calling convention as sys_symlink().

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>

fs/internal.h

@@ -58,6 +58,8 @@ extern int vfs_path_lookup(struct dentry *, struct vfsmount *,
 long do_mkdirat(int dfd, const char __user *pathname, umode_t mode);
 long do_rmdir(int dfd, const char __user *pathname);
 long do_unlinkat(int dfd, struct filename *name);
+long do_symlinkat(const char __user *oldname, int newdfd,
+		  const char __user *newname);
 
 /*
  * namespace.c

fs/namei.c

@@ -4113,8 +4113,8 @@ int vfs_symlink(struct inode *dir, struct dentry *dentry, const char *oldname)
 }
 EXPORT_SYMBOL(vfs_symlink);
 
-SYSCALL_DEFINE3(symlinkat, const char __user *, oldname,
-		int, newdfd, const char __user *, newname)
+long do_symlinkat(const char __user *oldname, int newdfd,
+		  const char __user *newname)
 {
 	int error;
 	struct filename *from;
@@ -4144,9 +4144,15 @@ out_putname:
 	return error;
 }
 
+SYSCALL_DEFINE3(symlinkat, const char __user *, oldname,
+		int, newdfd, const char __user *, newname)
+{
+	return do_symlinkat(oldname, newdfd, newname);
+}
+
 SYSCALL_DEFINE2(symlink, const char __user *, oldname, const char __user *, newname)
 {
-	return sys_symlinkat(oldname, AT_FDCWD, newname);
+	return do_symlinkat(oldname, AT_FDCWD, newname);
 }
 
 /**

include/linux/syscalls.h

@@ -979,4 +979,13 @@ static inline long ksys_mkdir(const char __user *pathname, umode_t mode)
 	return do_mkdirat(AT_FDCWD, pathname, mode);
 }
 
+extern long do_symlinkat(const char __user *oldname, int newdfd,
+			 const char __user *newname);
+
+static inline long ksys_symlink(const char __user *oldname,
+				const char __user *newname)
+{
+	return do_symlinkat(oldname, AT_FDCWD, newname);
+}
+
 #endif

init/initramfs.c

@@ -392,7 +392,7 @@ static int __init do_symlink(void)
 {
 	collected[N_ALIGN(name_len) + body_len] = '\0';
 	clean_path(collected, 0);
-	sys_symlink(collected + N_ALIGN(name_len), collected);
+	ksys_symlink(collected + N_ALIGN(name_len), collected);
 	sys_lchown(collected, uid, gid);
 	do_utime(collected, mtime);
 	state = SkipIt;