AS-safe change for multithreaded setxid breaks chroot() along with set*id()

[Lance Chen <cyen0312@gmail.com>]

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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
http://git.musl-libc.org/cgit/musl/commit/?id=78a8ef47c4d92b7680c52a85f80a81e29da86bb9,
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
https://gist.github.com/Lance0312/c7a82793e35b322bfbdfe96b79dd3152.

I understand how important it is to make set*id calls AS-safe, but there
might be a bunch of programs using set*id along with pthread and chroot.
(postfix could be one of them, I haven't take a deeper look at it though)
And I think it does not quite make sense to ask user not to use the
combination of the three features.

Does anyone have any possible fix for this? Or anyone know any WIP on
the Linux kernel to provide a truly atomic set*id syscall among threads?

Related thread: Fixing multithreaded set*id() AS-safety http://www.openwall.com/lists/musl/2014/12/20/1

Cheers,
Lance Chen

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