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

[Tycho Andersen]

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.

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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>