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Date: Wed, 3 Feb 2021 15:21:06 -0500
From: Dominic Chen <d.c.ddcc@...il.com>
To: Rich Felker <dalias@...c.org>, fweimer@...hat.com
Cc: musl@...ts.openwall.com
Subject: Re: Incorrect thread TID caching
On 2/3/2021 2:16 AM, Florian Weimer wrote:
> If you use the clone system call wrapper in threading (not fork/vfork)
> mode, you cannot call any libc functions afterwards, including the
> syscall function. Instead, you have to issue direct system calls.
On 2/3/2021 2:21 PM, Rich Felker wrote:
> Unfortunately it's really underdocumented and underexplored what a
> child created with clone() can do. There are definitely limitations --
> for example any usage with CLONE_VM or CLONE_THREAD is restricted not
> to call into libc at all, and might not even be safe whatsoever.
> However basic usage comparable in semantics to _Fork is probably
> supposed to work at least as well as _Fork -- in particular calling
> AS-safe libc functions should work.
I wasn't aware of this behavior, and didn't see any documentation about
this for the glibc clone() wrapper either. This seems to be a big
footgun, and after looking through the history for this code in Chrome,
it looks like they had a similar issue with glibc too.
> BTW does Chrom{e,ium} itself do something with raw clone? If so this
> could be a source of some of the bugs users hit, and it would be great
> to get a clearer picture on what's happening.
The code in question is a unittest for the sandbox, which manually calls
clone with CLONE_NEWPID to fork a child in a PID namespace, then
installs a signal handler and checks that it receives SIGTERM correctly:
https://source.chromium.org/chromium/chromium/src/+/master:sandbox/linux/services/namespace_sandbox_unittest.cc;l=194
. But under musl, raise() uses the cached TID value, so the test
eventually times out.
I missed that the NamespaceSandbox::ForkInNewPidNamespace() function
does manually update the cached TID for glibc after calling the
ForkWithFlags wrapper, so I can just do the same for musl too.
Separately, it looks like glibc used to have a PID cache too, but was
removed after a discussion that you were both involved with:
http://sourceware-org.1504.n7.nabble.com/Caching-of-PID-TID-after-fork-td416394.html
Thanks,
Dominic
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