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Date: Fri, 1 Apr 2016 21:03:12 -0400
From: Rich Felker <>
Subject: Re: AS-safe change for multithreaded setxid breaks chroot()
 along with set*id()

On Fri, Apr 01, 2016 at 08:28:48PM +0000, Josiah Worcester wrote:
> On Fri, Apr 1, 2016 at 1:23 PM Lance Chen <> wrote:
> > Hello there,
> >
> > I have been trying to run powerdns on Alpine Linux, which features musl,
> > without success. It turns out that the AS-safe strategy introduced in
> > the patch
> >
> >
> > ,
> > especially the use of reading /proc/self/task, causes the failure. The
> > failure condition is a combination of three features, pthread, chroot,
> > and set*id. After chroot()'ing to some directory, calling set*id() will
> > try to access /proc/self/task, and fail to find the directory in the new
> > root. I've created a minimal POC at
> >
> A fairly simple fix for this would be to create and mount /proc in the
> chroot. This is likely to be needed for some other interfaces in libc as
> well as other libraries and utilities, because there's functionality of
> Linux which is only really exposed in that filesystem. I don't know of
> anything in particular that would fail on glibc, but I am confident that
> you'll have issues, just like if you don't have /dev in the chroot.

Indeed. Due to Linux limitations, it's impossible to provide complete
behavior for some functions without /proc. In the case of set*id, the
right solution is to fix the kernel so that it can atomically change
the ids of a whole process, but I looked at doing that and it's hard
(because of a lot of architectural flaws on the kernel's part, like
using an allocated struct cred and RCU mess to represent ids rather
than just putting them inline in the task struct).


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