[Devel] [PATCH 00/25] Mount writer count and read-only bind mounts (v7)

[Dave Hansen]

I figured I'd run this past the containers list one more time.

These are against 2.6.19-mm1.  They compile, boot and pass my
little attached test script.

Changes from previous version
- rework helper names for doing custom 'struct file' creation

---

The following series implements read-only bind mounts.  This feature
allows a read-only view into a read-write filesystem.  In the process
of doing that, it also provides infrastructure for keeping track of
the number of writers to any given mount.  In this version, if there
are writers on a superblock, the filesystem may not be remounted 
r/o.  The same goes for MS_BIND mounts, and writers on a vfsmount.

This has a number of uses.  It allows chroots to have parts of
filesystems writable.  It will be useful for containers in the future
and is intended to replace patches that vserver has had out of the
tree for several years.  It allows security enhancement by making
sure that parts of your filesystem read-only, when you don't want
to have entire new filesystems mounted, or when you want atime
selectively updated.

This set makes no attempt to keep the return codes for these
r/o bind mounts the same as for a real r/o filesystem or device.
It would require significantly more code and be quite a bit more
invasive.

Using this feature requires two steps:

	mount --bind /source /dest
	mount -o remount,ro  /dest

I've been using the following script to test that the feature is
working as desired.  It takes a directory and makes a regular
bind and a r/o bind mount of it.  It then performs some normal
filesystem operations on the three directories, including ones
that are expected to fail, like creating a file on the r/o
mount.

#!/bin/sh
DIRS="foo foo-bound foo-bound-ro"
set -e

function do_umount
{
	(
	 umount foo-bound || true;
	 umount foo-bound-ro || true;
	) 2> /dev/null
}

trap do_umount ERR

# just in case the last invocation left them
do_umount

function should_fail
{
	"$@" > /dev/null 2>&1 \
	&& echo unexpected success: "$@" \
	|| echo GOOD: expected failure: "$@";
}
function should_succeed
{
	"$@" > /dev/null 2>&1 \
	&& echo GOOD: expected success: "$@" \
	|| echo unexpected failure: "$@"
}
function should_fail_ro
{
	RO=$1
	shift
	if $RO; then
		should_fail "$@"
	else
		should_succeed "$@"
	fi
}
function testdir
{
	RO=$1
	shift;
	i=$1
	should_fail_ro $RO touch $i/$i-file
	should_fail_ro $RO mkdir $i/$i-dir
	should_fail_ro $RO mknod $i/$i-null-chardev c 1 3 
	should_fail_ro $RO chmod 777 $i/$i-null-chardev
}
for i in $DIRS; do
	rm -r ./$i > /dev/null 2>&1 || true
	mkdir -p $i;
done;

mount --bind foo foo-bound/	 || exit
mount --bind foo foo-bound-ro	 || exit
mount -o remount,ro foo-bound-ro || exit

testdir false foo
testdir false foo-bound
testdir true  foo-bound-ro

should_succeed echo foo > foo-bound/foo-null-chardev
should_succeed echo foo > foo-bound-ro/foo-null-chardev

should_fail chmod 777 foo-bound-ro/foo-dir
should_fail chmod 777 foo-bound-ro/foo-file
should_fail chown nobody foo-bound-ro/foo-dir
should_fail chown nobody foo-bound-ro/foo-file
should_fail rmdir foo-bound-ro/foo-dir

do_umount

Signed-off-by: Dave Hansen <haveblue at us.ibm.com>
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