linux-kernel - Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

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Date:   Tue, 17 Jul 2018 10:40:41 +0100
From:   David Howells <dhowells@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     dhowells@...hat.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>
Subject: Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

Andy Lutomirski <luto@...capital.net> wrote:

> > Whilst I'm at it, do we want the option of doing the equivalent of
> > mountat()?  I.e. offering the option to open all the device files used by
> > a superblock with dfd and AT_* flags in combination with the filename?
> > 
> 
> Isn’t that more or less what I was suggesting?  I suggested dfd and path and I also suggested just an fd and letting the caller open the file itself.

Do we need AT_* flags?  There are three that we could use:

	AT_SYMLINK_NOFOLLOW
	AT_NO_AUTOMOUNT
	AT_EMPTY_PATH

AT_EMPTY_PATH I can see, but I don't see it as likely that we'd want to use
the other two for selecting a source?  Note that we can always do:

	fsfd = fsopen("ext4");
	sfd = open("/dev/", O_PATH);
	fsconfig(fsfd, fsconfig_set_path, "journal_path", "sda1", sfd);

or:

	fsfd = fsopen("ext4");
	sfd = open("/dev/sda1", O_PATH);
	fsconfig(fsfd, fsconfig_set_path_empty, "journal_path", "", sfd);

or:

	fsfd = fsopen("ext4");
	jfd = open("/dev/sda1", O_RDWR);
	fsconfig(fsfd, fsconfig_set_fd, "journal_path", NULL, jfd);

assuming the open on the latter doesn't exclude the use by the filesystem.

This way I don't need a second syscall or a 6-arg syscall to handle path
specification.

David