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Date: Wed, 24 Jun 2020 16:39:59 -0700
From: Sami Tolvanen <samitolvanen@google.com>
Subject: Re: [PATCH 17/22] arm64: vdso: disable LTO
Message-ID: <20200624233959.GB94769@google.com>
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To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>, Will Deacon <will@kernel.org>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, "Paul E. McKenney" <paulmck@kernel.org>, Kees Cook <keescook@chromium.org>, clang-built-linux <clang-built-linux@googlegroups.com>, Kernel Hardening <kernel-hardening@lists.openwall.com>, linux-arch <linux-arch@vger.kernel.org>, Linux ARM <linux-arm-kernel@lists.infradead.org>, Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>, linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>

On Wed, Jun 24, 2020 at 04:05:48PM -0700, Nick Desaulniers wrote:
> On Wed, Jun 24, 2020 at 2:52 PM Sami Tolvanen <samitolvanen@google.com> wrote:
> >
> > On Wed, Jun 24, 2020 at 01:58:57PM -0700, 'Nick Desaulniers' via Clang Built Linux wrote:
> > > On Wed, Jun 24, 2020 at 1:33 PM Sami Tolvanen <samitolvanen@google.com> wrote:
> > > >
> > > > Filter out CC_FLAGS_LTO for the vDSO.
> > >
> > > Just curious about this patch (and the following one for x86's vdso),
> > > do you happen to recall specifically what the issues with the vdso's
> > > are?
> >
> > I recall the compiler optimizing away functions at some point, but as
> > LTO is not really needed in the vDSO, it's just easiest to disable it
> > there.
> 
> Sounds fishy; with extern linkage then I would think it's not safe to
> eliminate functions.  Probably unnecessary for the initial
> implementation, and something we can follow up on, but always good to
> have an answer to the inevitable question "why?" in the commit
> message.

Sure. I can test this again with the current toolchain to see if there
are still problems.

Sami