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[Robert Collins <729338@xxxxxxxxxxxxxxxxxx>]

FWIW we see this in LXC containers when we use aufs to make ephemeral
containers. An easy trigger seems to be deleting a file in the rw layer
that exists in the ro layer.

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https://bugs.launchpad.net/bugs/729338

Title:
  yama hardlink restriction misbehaves under aufs

Status in “linux” package in Ubuntu:
  Triaged

Bug description:
  On the mythbuntu liveCD mysqld throws errors during renaming of
  database files because hardlink restrictions are incorrectly
  triggered:

  root@ubuntu:~# sudo -u mysql /bin/bash
  mysql@ubuntu:/root$ id
  uid=102(mysql) gid=105(mysql) groups=105(mysql)
  mysql@ubuntu:/root$ cd /var/lib/mysql/mysql
  mysql@ubuntu:/var/lib/mysql/mysql$ umask 0006
  mysql@ubuntu:/var/lib/mysql/mysql$ touch cow
  mysql@ubuntu:/var/lib/mysql/mysql$ ls -lda . user.MYI cow
  drwx------ 2 mysql root   140 2011-03-04 12:44 .
  -rw-rw---- 1 mysql mysql    0 2011-03-04 12:44 cow
  -rw-rw---- 1 mysql mysql 2048 2011-03-01 19:04 user.MYI
  mysql@ubuntu:/var/lib/mysql/mysql$ mv cow cow2
  mysql@ubuntu:/var/lib/mysql/mysql$ mv user.MYI user2
  mv: cannot move `user.MYI' to `user2': Operation not permitted
  mysql@ubuntu:/var/lib/mysql/mysql$ dmesg | tail -n1
  [ 6382.911220] non-accessible hardlink creation was attempted by: mv (fsuid 102)
  mysql@ubuntu:/var/lib/mysql/mysql$ cat user.MYI > cow2
  mysql@ubuntu:/var/lib/mysql/mysql$ ls -la cow2 user.MYI
  -rw-rw---- 1 mysql mysql 2048 2011-03-04 12:45 cow2
  -rw-rw---- 1 mysql mysql 2048 2011-03-01 19:04 user.MYI
  mysql@ubuntu:/var/lib/mysql/mysql$ lsattr cow2 user.MYI
  lsattr: Inappropriate ioctl for device While reading flags on cow2
  lsattr: Inappropriate ioctl for device While reading flags on user.MYI
  mysql@ubuntu:/var/lib/mysql/mysql$ df -h .
  Filesystem            Size  Used Avail Use% Mounted on
  aufs                  247M   28M  220M  12% /
  mysql@ubuntu:/var/lib/mysql/mysql$ grep aufs /proc/mounts 
  aufs / aufs rw,noatime,si=428526c014c98973 0 0
  mysql@ubuntu:/var/lib/mysql/mysql$ exit
  mysql@ubuntu:/var/lib/mysql/mysql# cd /sys/fs/aufs/si_428526c014c98973
  root@ubuntu:/sys/fs/aufs/si_428526c014c98973# ls -la
  total 0
  drwxr-xr-x 2 root root    0 2011-03-04 12:52 .
  drwxr-xr-x 3 root root    0 2011-03-04 12:52 ..
  -r--r--r-- 1 root root 4096 2011-03-04 12:52 br0
  -r--r--r-- 1 root root 4096 2011-03-04 12:52 br1
  -r--r--r-- 1 root root 4096 2011-03-04 12:52 xi_path
  root@ubuntu:/sys/fs/aufs/si_428526c014c98973# cat br0
  /cow=rw
  root@ubuntu:/sys/fs/aufs/si_428526c014c98973# cat br1
  /rofs=rr

  The main hardlink restriction check is this:
          if (cred->fsuid != inode->i_uid &&
              (!S_ISREG(mode) || (mode & S_ISUID) ||
               ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) ||
               (generic_permission(inode, MAY_READ | MAY_WRITE, 0, NULL))) &&
              !capable(CAP_FOWNER)) {
  or, in english:
   * Block hardlink when all of:
   *  - fsuid does not match inode
   *  - not CAP_FOWNER
   *  - and at least one of:
   *    - inode is not a regular file
   *    - inode is setuid
   *    - inode is setgid and group-exec
   *    - access failure for read and write

  nothing visible in userspace seems to violate this, so this looks like
  some kind of aufs issue where the exposed inode does not match the
  toplevel visible to userspace? I'm not sure yet what's going on.

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