linux-kernel - Re: [PATCH] mips: vdso: conditionalize 32-bit time functions on COMPAT_32BIT

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Date:   Mon, 30 Dec 2019 13:34:00 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        Paul Burton <paulburton@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Christian Brauner <christian.brauner@...onical.com>
Subject: Re: [PATCH] mips: vdso: conditionalize 32-bit time functions on COMPAT_32BIT_TIME

On Mon, Dec 30, 2019 at 1:27 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> On Mon, Dec 30, 2019 at 12:58 PM Arnd Bergmann <arnd@...db.de> wrote:
> > Thanks for the bug report! I'm not completely sure why this fails in
> > this particular
> > way though. I assume you are using musl-1.1.20, not a musl-1.2.0 snapshot
>
> Yes, that's the one, sorry.
>
> > diff --git a/arch/mips/vdso/vdso.lds.S b/arch/mips/vdso/vdso.lds.S
> > index da4627430aba..0bdc6a026be8 100644
> > --- a/arch/mips/vdso/vdso.lds.S
> > +++ b/arch/mips/vdso/vdso.lds.S
> > @@ -93,9 +93,11 @@ VERSION
> >         LINUX_2.6 {
> >  #ifndef DISABLE_MIPS_VDSO
> >         global:
> > +#if (_MIPS_SIM == _MIPS_SIM_ABI64) || defined(CONFIG_COMPAT_32BIT_TIME)
> >                 __vdso_clock_gettime;
> >                 __vdso_gettimeofday;
> >                 __vdso_clock_getres;
> > +#endif
> >  #if _MIPS_SIM != _MIPS_SIM_ABI64
> >                 __vdso_clock_gettime64;
> >  #endif
> >
> > That should ensure that no user space can call the old vdso
> > functions on a kernel that intentionally breaks the actual
> > syscalls.
>
> I can confirm that patch fixes things. Thanks.

Ok, that's good news, but I think we still need to answer two questions:

- Why does it crash in the first place rather than returning -ENOSYS?

- How does it actually work if you run an application built against
  an old musl version on a kernel that tries to make this not work?
  Do you just get a random time (uninitialized user space stack) and
  work with that without checking the error code?

      Arnd