linux-kernel - Re: [PATCH 4/9] Implement fsopen() to prepare for a mount

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Message-ID: <15169.1493903211@warthog.procyon.org.uk>
Date:   Thu, 04 May 2017 14:06:51 +0100
From:   David Howells <dhowells@...hat.com>
To:     Karel Zak <kzak@...hat.com>
Cc:     dhowells@...hat.com, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, mszeredi@...hat.com
Subject: Re: [PATCH 4/9] Implement fsopen() to prepare for a mount

Karel Zak <kzak@...hat.com> wrote:

> > 	write(mfd, "d /dev/sdb1"); // note I'm ignoring write's length arg
> 
> Not sure about 'd', in many cases it is not device, for mount(2)
> syscall we call it "source".

sys_mount() calls it devname.  But whatever - I'm not particularly attached to
the letter 'd' for this.

> The very basic mount(2) problem is that you have to parse
> /proc/self/mountinfo to get information about the mounted filesystem.
> It seems that your read() is also one way communication.
> 
> What we really need is to have a way how to specify *what* you want to
> read. The error message is not enough, I want to know the finally used
> mount options, mount ID, etc. It would be nice to have something like
> 
> 
>    fsmount(mfd, AT_FDCWD, "/mnt", 0);
> 
>    write(mfd, "o");
>    read(mfd, ....);     // read mount options
> 
>    write(mdf, "i");
>    read(mfd, ....);     // read mount ID
> 
> 
> but it seems ugly. Maybe introduce another function like 
> 
>     fsinfo(mdf, "o", buf, bufsz)
> 
> to get mount options (etc.) and to avoid separate write & read.

What is it you're trying to do?  Just read back the state of the new mount?
Or read back the state of a specified extant mount?

David