linux-kernel - Re: [PATCH] arm64: Allow disabling of the compat vDSO

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Date:   Thu, 26 Sep 2019 09:40:43 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Vincenzo Frascino <vincenzo.frascino@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: Allow disabling of the compat vDSO

On Thu, Sep 26, 2019 at 8:51 AM Catalin Marinas <catalin.marinas@....com> wrote:
>
> On Thu, Sep 26, 2019 at 08:47:18AM +0100, Catalin Marinas wrote:
> > On Thu, Sep 26, 2019 at 01:06:50AM +0100, Vincenzo Frascino wrote:
> > > On 9/25/19 6:08 PM, Catalin Marinas wrote:
> > > > On Wed, Sep 25, 2019 at 09:53:16AM -0700, Nick Desaulniers wrote:
> > > >> On Wed, Sep 25, 2019 at 6:09 AM Catalin Marinas <catalin.marinas@....com> wrote:
> > > >>> - clean up the headers includes; vDSO should not include kernel-only
> > > >>>   headers that may even contain code patched at run-time
> > > >>
> > > >> This is a big one; Clang validates the inline asm constraints for
> > > >> extended inline assembly, GCC does not for dead code.  So Clang chokes
> > > >> on the inclusion of arm64 headers using extended inline assembly when
> > > >> being compiled for arm-linux-gnueabi.

This case is very much real (not sure if Vincenzo was asking me or
Catalin), see report at the bottom of this comment:
https://github.com/ClangBuiltLinux/linux/issues/595#issuecomment-509874891

> > > >
> > > > Whether clang or gcc, I'd like this fixed anyway. At some point we may
> > > > inadvertently rely on some code which is patched at boot time for the
> > > > kernel code but not for the vDSO.
> > >
> > > Do we have any code of this kind in header files?
> > >
> > > The vDSO library uses only a subset of the headers (mainly Macros) hence all the
> > > unused symbols should be compiled out. Is your concern only theoretical or do
> > > you have an example on where this could be happening?
> >
> > At the moment it's rather theoretical.
>
> Actually, it's not. The moment the compat vdso Makefile needs the line
> below, we are doing it wrong:
>
> VDSO_CFLAGS += -D__uint128_t='void*'

*yikes*
-- 
Thanks,
~Nick Desaulniers