It would be preferable to do a mount --bind --make-private in one atomic action [ASCII art this time]

Anders Blomdell
2011-11-23T17:36:38Z

Sorry about previous posts used character set!

The rationale is the race problems I recently found with pam_namspace
(see https://bugzilla.redhat.com/show_bug.cgi?id=755216). The following
small script and it's output shows what pam_namespace essentially does
does, and the problems that might occur; assume that /work is
automounted, then the following really confuses things:

(
echo '# Empty work'
findmnt -a | cut -c1-30 | grep work
echo '# First primary mount on work'
ls -ld /work/Fedora-16/.
findmnt -a | cut -c1-30 | grep work
unshare --mount -- /bin/sh -c "(
mount --bind /work /work ;
mount --make-private /work ;
mount --bind /tmp /work ;
echo '# Detached mount of work' ;
findmnt -a | cut -c1-30 | grep work ;
sleep 5 ;
echo '# Before detached unmount of private work' ;
findmnt -a | cut -c1-30 | grep work;
umount /work ;
echo '# Before detached unmount of bound work' ;
findmnt -a | cut -c1-30 | grep work;
echo '# Weird detached automount behaviour' ;
ls -ld /work/Fedora-15/. ;
umount /work ;
echo '# Detached unmount') &"
sleep 1
echo '# Second primary mount on work'
ls -ld /work/Fedora-15/.
findmnt -a | cut -c1-30 | grep work
echo '# First primary mount on work no longer accessible'
ls -ld /work/Fedora-16/.
sleep 10
echo '# And the final remaining cruft'
findmnt -a | cut -c1-30 | grep work
)

Which gives the following output (no wonder I had problems understanding
what went wrong with my machines :-():

# Empty work
|-/work
# First primary mount on work
drwxr-xr-x 4 root root 4096 Nov 23 17:59 /work/Fedora-16/.
|-/work
| `-/work/Fedora-16
# Detached mount of work
|-/work
| |-/work/Fedora-16
| `-/work
| `-/work
# Second primary mount on work
drwxr-xr-x 4 root root 4096 Nov 23 17:59 /work/Fedora-15/.
|-/work
| |-/work/Fedora-16
| |-/work
| | `-/work/Fedora-15
| `-/work/Fedora-15
# First primary mount on work no longer accessible
ls: cannot access /work/Fedora-16/.: Too many levels of symbolic links
# Before detached unmount of private work
|-/work
| |-/work/Fedora-16
| |-/work
| | `-/work
| `-/work/Fedora-15
# Before detached unmount of bound work
|-/work
| |-/work/Fedora-16
| |-/work
| `-/work/Fedora-15
# Weird detached automount behaviour
ls: cannot access /work/Fedora-15/.: Too many levels of symbolic links
# Detached unmount
# And the final remaining cruft
|-/work
| |-/work/Fedora-16
| |-/work
| | `-/work/Fedora-15
| `-/work/Fedora-15

--
Anders Blomdell Email: anders.blomdell@control.lth.se
Department of Automatic Control
Lund University Phone: +46 46 222 4625
P.O. Box 118 Fax: +46 46 138118
SE-221 00 Lund, Sweden