[CRIU] [PATCH v2 3/3] mount: handle tracefs more gracefully

[Tycho Andersen]

On Wed, May 11, 2016 at 08:49:19AM -0600, Tycho Andersen wrote:
> On Wed, May 11, 2016 at 03:49:07PM +0300, Pavel Emelyanov wrote:
> > On 05/11/2016 12:15 AM, Tycho Andersen wrote:
> > > See the comment for details, but basically tracefs is automounted by the
> > > kernel, so we can just mount debugfs with MS_REC and get the right result.
> > > 
> > > v2: rebase on criu-dev
> > > 
> > > Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
> > > ---
> > > This is kind of similarly ugly to to a cgroup patch I sent earlier, where
> > > we needed some filesystem specific code and checks in various parts of
> > > mount.c. I somehow can't figure out a nicer way to do this, but I am
> > > definitely open to suggestions.
> > 
> > A suggestion :)
> > 
> > > @@ -2301,6 +2310,11 @@ static int do_new_mount(struct mount_info *mi)
> > >  	if (remount_ro)
> > >  		sflags &= ~MS_RDONLY;
> > >  
> > > +	if (tp->flags) {
> > > +		pr_info("forcing flags: %d\n", tp->flags);
> > > +		sflags |= tp->flags;
> > > +	}
> > > +
> > >  	if (do_mount(mi, src, mnt_fsname(mi), sflags) < 0) {
> > 
> > This do_mount can call mi->fs->mount() callback. What if we define one
> > for debugfs that would just call do_simple_mount() with updated flags?
> 
> Yes, this sounds a lot better :). I'll resend, thanks for the
> suggestion.

Ugh, actually this won't work for us, because liblxc by default bind
mounts debugfs into the container, rather than letting the container
mount it itself, so the hunk we actually care about for this patch is
the bit in do_bind:

@@ -2474,12 +2488,16 @@ do_bind:
                }
        }

-       if (mount(root, mi->mountpoint, NULL, MS_BIND, NULL) < 0) {
+       if (mount(root, mi->mountpoint, NULL, MS_BIND | mi->fstype->flags, NULL) < 0) {
                pr_perror("Can't mount at %s", mi->mountpoint);
                goto err;
        }

        mflags = mi->flags & (~MS_PROPAGATE);
+       if (mi->fstype->flags) {
+               pr_info("forcing flags: 0x%x\n", mi->fstype->flags);
+               mflags |= mi->fstype->flags;
+       }
        if (!mi->bind || mflags != (mi->bind->flags & (~MS_PROPAGATE)))
                if (mount(NULL, mi->mountpoint, NULL, MS_BIND | MS_REMOUNT | mflags, NULL)) {
                        pr_perror("Can't mount at %s", mi->mountpoint);

So we need some way to hook that. Maybe adding another fstype->bind hook, so
it's more general than just flags, similar to the ->mount hook above?

Tycho