linux-kernel - Re: [PATCH RESEND v1 3/4] powerpc/vdso: Separate vvar vma from vdso

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Date:   Sun, 4 Apr 2021 22:03:37 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        dima@...sta.com, arnd@...db.de, tglx@...utronix.de,
        vincenzo.frascino@....com, luto@...nel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH RESEND v1 3/4] powerpc/vdso: Separate vvar vma from vdso

On Wed, Mar 31, 2021 at 04:48:46PM +0000, Christophe Leroy wrote:
> From: Dmitry Safonov <dima@...sta.com>
> 
> Since commit 511157ab641e ("powerpc/vdso: Move vdso datapage up front")
> VVAR page is in front of the VDSO area. In result it breaks CRIU
> (Checkpoint Restore In Userspace) [1], where CRIU expects that "[vdso]"
> from /proc/../maps points at ELF/vdso image, rather than at VVAR data page.
> Laurent made a patch to keep CRIU working (by reading aux vector).
> But I think it still makes sence to separate two mappings into different
> VMAs. It will also make ppc64 less "special" for userspace and as
> a side-bonus will make VVAR page un-writable by debugger (which previously
> would COW page and can be unexpected).
> 
> I opportunistically Cc stable on it: I understand that usually such
> stuff isn't a stable material, but that will allow us in CRIU have
> one workaround less that is needed just for one release (v5.11) on
> one platform (ppc64), which we otherwise have to maintain.
> I wouldn't go as far as to say that the commit 511157ab641e is ABI
> regression as no other userspace got broken, but I'd really appreciate
> if it gets backported to v5.11 after v5.12 is released, so as not
> to complicate already non-simple CRIU-vdso code. Thanks!
> 
> Cc: Andrei Vagin <avagin@...il.com>

Acked-by: Andrei Vagin <avagin@...il.com>

> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> Cc: Christophe Leroy <christophe.leroy@...roup.eu>
> Cc: Laurent Dufour <ldufour@...ux.ibm.com>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: linuxppc-dev@...ts.ozlabs.org
> Cc: stable@...r.kernel.org # v5.11
> [1]: https://github.com/checkpoint-restore/criu/issues/1417
> Signed-off-by: Dmitry Safonov <dima@...sta.com>
> Tested-by: Christophe Leroy <christophe.leroy@...roup.eu>
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>