[CRIU] [PATCH] mount: Create target of bind mount if it doesn't exist

[Tycho Andersen]

On Thu, Aug 28, 2014 at 11:17:16AM -0500, Tycho Andersen wrote:
> On Thu, Aug 28, 2014 at 08:07:06PM +0400, Pavel Emelyanov wrote:
> > On 08/28/2014 07:43 PM, Saied Kazemi wrote:
> > > I have seen a similar case where /dev/null is missing (Docker using UnionFS).  While this patch fixes
> > > the issue, I am wondering why the target file should be missing to start with and whether *always*
> > > recreating missing target files could be masking more serious issues.  Any thoughts?
> > 
> > +1  I'd also like to know the reasons why the mountpoint target doesn't exist.
> > AFAIK rmdir/unlink doesn't work on dirs/files that are mountpoints.
> 
> They don't exist because rsync doesn't sync them :). What I'm doing
> here is dumping a container, rsyncing over its rootfs and the criu
> output, and then trying to restore it on another host. I'm expecting
> criu to restore from there (including mounting udev, which is
> presumably why /dev/null is missing). I guess mounting udev would also
> fix this problem, though? I'm not sure.

Urgh. After talking with Serge, he points out that I needed to add
--devices to my rsync to pick these up :). Doing that obviates the
need for this patch on my end. Sorry for the noise.

Tycho

> Tycho
> 
> > > --Saied
> > > 
> > > 
> > > 
> > > On Thu, Aug 28, 2014 at 8:27 AM, Tycho Andersen <tycho.andersen at canonical.com <mailto:tycho.andersen at canonical.com>> wrote:
> > > 
> > >     If the target of a bind mount doesn't exist, create it. In particular, this
> > >     happens with /dev/ptmx across hosts under LXC. We could also mkdir -p any path
> > >     to the target, although I'm not sure if that is a case that occurrs in the
> > >     wild.
> > > 
> > >     Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com <mailto:tycho.andersen at canonical.com>>
> > >     ---
> > >      mount.c | 18 ++++++++++++++++++
> > >      1 file changed, 18 insertions(+)
> > > 
> > >     diff --git a/mount.c b/mount.c
> > >     index 9bb8a17..2108907 100644
> > >     --- a/mount.c
> > >     +++ b/mount.c
> > >     @@ -1346,6 +1346,24 @@ static int do_bind_mount(struct mount_info *mi)
> > >                     root = rpath;
> > >      do_bind:
> > >                     pr_info("\tBind %s to %s\n", root, mi->mountpoint);
> > >     +
> > >     +               if (access(mi->mountpoint, F_OK)) {
> > >     +                       if (errno == ENOENT) {
> > >     +                               FILE *f;
> > >     +
> > >     +                               f = fopen(mi->mountpoint, "w");
> > >     +                               if (!f) {
> > >     +                                       pr_perror("couldn't write 0 length %s", mi->mountpoint);
> > >     +                                       return -1;
> > >     +                               }
> > >     +
> > >     +                               fclose(f);
> > >     +                       } else {
> > >     +                               pr_perror("Couldn't access %s", mi->mountpoint);
> > >     +                               return -1;
> > >     +                       }
> > >     +               }
> > >     +
> > >                     if (mount(root, mi->mountpoint, NULL,
> > >                                             MS_BIND, NULL) < 0) {
> > >                             pr_perror("Can't mount at %s", mi->mountpoint);
> > >     --
> > >     1.9.1
> > > 
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> > > 
> > > 
> > > 
> > > 
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> >