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Date: Thu, 13 Jan 2022 22:57:20 +0100
From: Florian Weimer <>
To: Andy Lutomirski <>
Cc:,  Linux API <>,,,,  the arch/x86 maintainers <>,,,,  Dave Hansen <>,  Kees
 Cook <>,  Andrei Vagin <>
Subject: Re: [PATCH v3 3/3] x86: Add test for arch_prctl(ARCH_VSYSCALL_CONTROL)

* Andy Lutomirski:

> On 1/5/22 08:03, Florian Weimer wrote:
>> Signed-off-by: Florian Weimer <>
> This seems like a respectable test case, but why does it work so hard
> to avoid using libc?

Back when this was still a true lockout and not a toggle, it was
necessary to bypass the startup code, so that the test still works once
the (g)libc startup starts activating the lockout.  The /proc mounting
is there to support running as init in a VM (which makes development so
much easier).

I could ditch the /proc mounting, perform some limited data gathering in
a pre-_start routine, undo a potential lockout before the tests, and
then use libc functions for the actual test.  It would probably be a bit
less code (printf is nice), but I'd probably have to use direct system
calls for the early data gathering anyway, so those parts would still be


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