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

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Date:   Mon, 30 Dec 2019 16:39:41 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Arnd Bergmann <arnd@...db.de>
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 4:37 PM Arnd Bergmann <arnd@...db.de> wrote:
>
> On Mon, Dec 30, 2019 at 3:37 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
> >
> > On Mon, Dec 30, 2019 at 1:34 PM Arnd Bergmann <arnd@...db.de> wrote:
> > >
> > > - Why does it crash in the first place rather than returning -ENOSYS?
> >
> > There's a bit of speculation about this in the original thread that
> > prompted this patch (you're CC'd).
> >
> > >
> > > - 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?
> >
> > Actually, your patch fails here. The ts struct remains as it was
> > before, filled with garbage. No good. My original patch in this
> > thread, though, does result in the correct value being written to ts.
>
> Ok, that is the intended behavior then, clock_gettime() needs
> to fail with -EINVAL or -ENOSYS here (depending on the libc
> implementation), and of course the data is not updated.
>
> Returning success from clock_gettime() on a kernel with only
> time64 support and a libc with only time32 support (or vice
> versa) would be a bug.

Ah, right, hence why the 32-bit compat code is behind a
still-on-by-default-but-not-for-long menu option.