linux-kernel - Re: RFC: permit link(2) to work across --bind mounts ?

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Message-ID: <20071220013355.3900.qmail@science.horizon.com>
Date:	19 Dec 2007 20:33:55 -0500
From:	linux@...izon.com
To:	linux-kernel@...r.kernel.org, lkml@....ca
Subject: Re: RFC: permit link(2)  to work across --bind mounts ?

> Why does link(2) not support hard-linking across bind mount points
> of the same underlying filesystem ?

Whenever we get mount -r --bind working properly (which I use to place
copies of necessary shared libraries inside chroot jails while allowing
page cache sharing), this feature would break security.

mkdir /usr/lib/libs.jail
for i in $LIST_OF_LIBRARIES; do
	ln /usr/lib/$i /usr/lib/libs.jail/$i
done
mount -r /usr/lib/libs.jail /jail/lib
chown prisoner /usr/log/jail
mount /usr/log/jail /jail/usr/log
chrootuid /jail prisoner /bin/untrusted &

Although protections should be enough, but I'd rather avoid having the
prisoner link /jail/lib/libfoo.so (write returns EROFS) to /jail/usr/log
where it's potentially writeable.
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