/proc does not get umounted after debootstrap

I can't reproduce this. Can you provide a debootstrap command (not in the context of pbuilder) that demonstrates this?

jean-louis@laptop-jl:~$ mkdir test
jean-louis@laptop-jl:~$ cd test
jean-louis@laptop-jl:~/test$ sudo debootstrap --include=apt --variant=buildd --keyring=/usr/share/keyrings/ubuntu-archive-keyring.gpg oneiric . http://archive.ubuntu.com/ubuntu/
[sudo] password for jean-louis:
I: Retrieving Release
I: Retrieving Release.gpg
I: Checking Release signature
I: Valid Release signature (key id 630239CC130E1A7FD81A27B140976EAF437D05B5)
I: Retrieving Packages
I: Validating Packages
I: Resolving dependencies of required packages...
I: Resolving dependencies of base packages...
I: Checking component main on http://archive.ubuntu.com/ubuntu...
I: Retrieving adduser
I: Validating adduser
I: Retrieving apt
I: Validating apt
I: Retrieving base-files
I: Validating base-files
I: Retrieving base-passwd
I: Validating base-passwd
I: Retrieving bash
I: Validating bash
I: Retrieving binutils
I: Validating binutils
I: Retrieving bsdutils
I: Validating bsdutils
I: Retrieving build-essential
I: Validating build-essential
I: Retrieving busybox-initramfs
I: Validating busybox-initramfs
I: Retrieving bzip2
I: Validating bzip2
I: Retrieving coreutils
I: Validating coreutils
I: Retrieving cpio
I: Validating cpio
I: Retrieving cpp
I: Validating cpp
I: Retrieving cpp-4.6
I: Validating cpp-4.6
I: Retrieving dash
I: Validating dash
I: Retrieving debconf
I: Validating debconf
I: Retrieving debianutils
I: Validating debianutils
I: Retrieving diffutils
I: Validating diffutils
I: Retrieving dpkg
I: Validating dpkg
I: Retrieving dpkg-dev
I: Validating dpkg-dev
I: Retrieving e2fslibs
I: Validating e2fslibs
I: Retrieving e2fsprogs
I: Validating e2fsprogs
I: Retrieving fakeroot
I: Validating fakeroot
I: Retrieving findutils
I: Validating findutils
I: Retrieving g++
I: Validating g++
I: Retrieving g++-4.6
I: Validating g++-4.6
I: Retrieving gcc
I: Validating gcc
I: Retrieving gcc-4.6
I: Validating gcc-4.6
I: Retrieving gcc-4.6-base
I: Validating gcc-4.6-base
I: Retrieving gnupg
I: Validating gnupg
I: Retrieving gpgv
I: Validating gpgv
I: Retrieving grep
I: Validating grep
I: Retrieving gzip
I: Validating gzip
I: Retrieving hostname
I: Validating hostname
I: Retrieving ifupdown
I: Validating ifupdown
I: Retrieving initramfs-tools
I: Validating initramfs-tools
I: Retrieving initramfs-tools-bin
I: Validating initramfs-tools-bin
I: Retrieving initscripts
I: Validating initscripts
I: Retrieving insserv
I: Validating insserv
I: Retrieving klibc-utils
I: Validating klibc-utils
I: Retrieving libacl1
I: Validating libacl1
I: Retrieving libalgorithm-diff-perl
I: Validating libalgorithm-diff-perl
I: Retrieving libalgorithm-diff-xs-perl
I: Validating libalgorithm-diff-xs-perl
I: Retrieving libalgorithm-merge-perl
I: Validating libalgorithm-merge-perl
I: Retrieving libattr1
I: Validating libattr1
I: Retrieving libblkid1
I: Validating libblkid1
I: Retrieving libbz2-1.0
I: Validating libbz2-1.0
I: Retrieving libc-bin
I: Validating libc-bin
I: Retrieving libc-dev-bin
I: Validating libc-dev-bin
I: Retrieving libc6
I: Validating libc6
I: Retrieving libc6-dev
I: Validating libc6-dev
I: Retrieving libclass-isa-perl
I: Validating libclass-isa-perl
I: Retrieving libcloog-ppl0
I: Validating libcloog-ppl0
I...

readlink("/media/mybook/chroot/805886/proc", 0xbfb669cb, 4096) = -1 EINVAL (Invalid argument)
stat64("/sbin/umount.proc", 0xbfb67878) = -1 ENOENT (No such file or directory)
rt_sigprocmask(SIG_BLOCK, ~[TRAP SEGV RTMIN RT_1], NULL, 8) = 0
oldumount("/proc") = -1 EBUSY (Device or resource busy)

The phrase "eh?" comes to mind.

Downgrading 'mount' to 2.17.2-9.1ubuntu4 fixes the issue, so reassigning to util-linux.

Could this be related to the high profile yet extremely uncommon pre-linux 3.0 bug around dentry cache being corrupted?

https://lwn.net/Articles/452138/
https://lwn.net/Articles/452141/

Raised to "High" as the pbuilder is an important tool for Ubuntu packagers.

I think that this is related to this behaviour on debootstrap exit, when it tries to umount the chrooted /proc:

root@oneiric:/var/cache/pbuilder/build/26972# umount /var/cache/pbuilder/build//26972/./proc
umount: /proc: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))

but the very odd behaviour is that if i try to do (changing working dir):

root@oneiric:/var/cache/pbuilder/build/26972# cd ..
root@oneiric:/var/cache/pbuilder/build# umount /var/cache/pbuilder/build//26972/./proc
root@oneiric:/var/cache/pbuilder/build#

Using mount 2.17.2-9.1ubuntu4 there are no issues.

The mounting is done via chroot, so the entry in /etc/mtab isn't present.

I think that it's more related to http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=634107

this is fixed now

util-linux (2.19.1-2ubuntu3) oneiric; urgency=low

  * Use canonicalize_spec in getmntdevbackward. proc should not be
    interpreted as a non-canonical-path. Thanks Francesco Del Degan!