linux-kernel - Re: [PATCH v3 3/3] x86: Add test for arch_prctl(ARCH_VSYSCALL

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Message-ID: <87pmovxprz.fsf@oldenburg.str.redhat.com>
Date:   Thu, 13 Jan 2022 22:57:20 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     linux-arch@...r.kernel.org, Linux API <linux-api@...r.kernel.org>,
        linux-x86_64@...r.kernel.org, kernel-hardening@...ts.openwall.com,
        linux-mm@...ck.org, the arch/x86 maintainers <x86@...nel.org>,
        musl@...ts.openwall.com, libc-alpha@...rceware.org,
        linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Andrei Vagin <avagin@...il.com>
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 <fweimer@...hat.com>
>
> 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
there.

Thanks,
Florian