linux-kernel - Re: [PATCH 17/22] arm64: vdso: disable LTO

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Date:   Wed, 24 Jun 2020 16:05:48 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linux-pci@...r.kernel.org,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>
Subject: Re: [PATCH 17/22] arm64: vdso: disable LTO

On Wed, Jun 24, 2020 at 2:52 PM Sami Tolvanen <samitolvanen@...gle.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@...gle.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.
-- 
Thanks,
~Nick Desaulniers