hostfs: support rename flags

Support RENAME_NOREPLACE and RENAME_EXCHANGE flags on hostfs if the
underlying filesystem supports it.

Since renameat2(2) is not yet in any libc, use syscall(2) to invoke the
renameat2 syscall.

Signed-off-by: Miklos Szeredi <[e-mail]>
Cc: Richard Weinberger <[e-mail]>
Signed-off-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/hostfs/hostfs.h

···
 extern int link_file(const char *from, const char *to);
 extern int hostfs_do_readlink(char *file, char *buf, int size);
 extern int rename_file(char *from, char *to);
+extern int rename2_file(char *from, char *to, unsigned int flags);
 extern int do_statfs(char *root, long *bsize_out, long long *blocks_out,
 		     long long *bfree_out, long long *bavail_out,
 		     long long *files_out, long long *ffree_out,

fs/hostfs/hostfs_kern.c

···
 	return err;
 }
 
-static int hostfs_rename(struct inode *from_ino, struct dentry *from,
-			 struct inode *to_ino, struct dentry *to)
+static int hostfs_rename2(struct inode *old_dir, struct dentry *old_dentry,
+			  struct inode *new_dir, struct dentry *new_dentry,
+			  unsigned int flags)
 {
-	char *from_name, *to_name;
+	char *old_name, *new_name;
 	int err;
 
-	if ((from_name = dentry_name(from)) == NULL)
+	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE))
+		return -EINVAL;
+
+	old_name = dentry_name(old_dentry);
+	if (old_name == NULL)
 		return -ENOMEM;
-	if ((to_name = dentry_name(to)) == NULL) {
-		__putname(from_name);
+	new_name = dentry_name(new_dentry);
+	if (new_name == NULL) {
+		__putname(old_name);
 		return -ENOMEM;
 	}
-	err = rename_file(from_name, to_name);
-	__putname(from_name);
-	__putname(to_name);
+	if (!flags)
+		err = rename_file(old_name, new_name);
+	else
+		err = rename2_file(old_name, new_name, flags);
+
+	__putname(old_name);
+	__putname(new_name);
 	return err;
 }
 
···
 	.mkdir		= hostfs_mkdir,
 	.rmdir		= hostfs_rmdir,
 	.mknod		= hostfs_mknod,
-	.rename		= hostfs_rename,
+	.rename2	= hostfs_rename2,
 	.permission	= hostfs_permission,
 	.setattr	= hostfs_setattr,
 };

fs/hostfs/hostfs_user.c

···
 #include <sys/time.h>
 #include <sys/types.h>
 #include <sys/vfs.h>
+#include <sys/syscall.h>
 #include "hostfs.h"
 #include <utime.h>
 
···
 	if (err < 0)
 		return -errno;
 	return 0;
+}
+
+int rename2_file(char *from, char *to, unsigned int flags)
+{
+	int err;
+
+#ifndef SYS_renameat2
+#  ifdef __x86_64__
+#    define SYS_renameat2 316
+#  endif
+#  ifdef __i386__
+#    define SYS_renameat2 353
+#  endif
+#endif
+
+#ifdef SYS_renameat2
+	err = syscall(SYS_renameat2, AT_FDCWD, from, AT_FDCWD, to, flags);
+	if (err < 0) {
+		if (errno != ENOSYS)
+			return -errno;
+		else
+			return -EINVAL;
+	}
+	return 0;
+#else
+	return -EINVAL;
+#endif
 }
 
 int do_statfs(char *root, long *bsize_out, long long *blocks_out,