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SubjectRe: [PATCH 4/9] Implement fsopen() to prepare for a mount
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On Thu, 2017-05-04 at 15:34 +0200, Karel Zak wrote:
> On Thu, May 04, 2017 at 02:06:51PM +0100, David Howells wrote:
> > Karel Zak <kzak@redhat.com> wrote:
> > > 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)
> > >

Not sure that it's any prettier, but you could use an ioctl to switch
between different read requests:

ioctl(mfd, FSOPEN_READ_OPTIONS);
read(mfd, ....);

ioctl(mfd, FSOPEN_READ_MOUNT_ID);
read(mfd, ....);

> > > 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?
>
> ...read back the state of the new mount, because for example mount
> options can be modified by FS driver. It would be also nice to have
> API to get state of arbitrary mount without parsing mountinfo (the
> file is huge on some systems).
>


--
Jeff Layton <jlayton@redhat.com>

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