Comment 0 for bug 1408106

With the following use of overlayfs, we get a disconnected path:

$ cat ./profile
#include <tunables/global>
profile foo {
  #include <abstractions/base>

  capability sys_admin,
  capability sys_chroot,
  mount,
  pivot_root,
}

$ cat ./overlay.c
#include <alloca.h>
#include <linux/sched.h>
#include <stdio.h>
#include <string.h>
#include <sys/mount.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char* argv[]) {
    int i = 0;
    int len = 0;
    int ret = 0;
    char* options;

    if (geteuid())
        unshare(CLONE_NEWUSER);
    unshare(CLONE_NEWNS);

    for (i = 1; i < argc; i++) {
        if (i == 1) {
            len = strlen(argv[i]) + strlen("upperdir=,lowerdir=/") + 2;
            options = alloca(len);
            ret = snprintf(options, len, "upperdir=%s,lowerdir=/", argv[i]);
        }
        else {
            len = strlen(argv[i]) + strlen("upperdir=,lowerdir=/mnt") + 2;
            options = alloca(len);
            ret = snprintf(options, len, "upperdir=%s,lowerdir=/mnt", argv[i]);
        }

        mount("overlayfs", "/mnt", "overlayfs", MS_MGC_VAL, options);
    }

    chdir("/mnt");
    pivot_root(".", ".");
    chroot(".");

    chdir("/");
    execl("/bin/bash", "/bin/bash", NULL);
}

$ sudo apparmor_parser -r ./profile && aa-exec -p foo -- ./a.out /tmp
[255]
...
Dec 12 14:31:38 localhost kernel: [57278.040216] audit: type=1400 audit(1418387498.613:712): apparmor="DENIED" operation="exec" info="Failed name lookup - disconnected path" error=-13 profile="foo" name="/bin/bash" pid=18255 comm="a.out" requested_mask="x" denied_mask="x" fsuid=1000 ouid=0

With the above, the expectation was for the denial to be /tmp/bin/bash. There are three ways forward:
1. the correct solution is to patch overlayfs to properly track the loopback, but this will take a while, may ultimately be unachievable (investigation is needed) and is likely non-upstreamable
2. we could rely on the fact that overlayfs creates a private unshared submount, and provide a way to not mediate the path when that is present, and tagged. This would take a bit of time, and might be the preferred method over 1 longer term
3. we could extend attach_disconnected so that we can define the attach root. Eg, we can use profile foo (attach_disconnected=/tmp) {} such that '/bin/bash' maps to '/tmp/bin/bash'.

While attach_disconnected should in general be discouraged, this method:
 * is doable in a short time frame,
 * is generally useful even when the proper fix is in place
 * would help lxc in a few cases
 * would be sufficient for snappy