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

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Date:   Mon, 30 Dec 2019 16:10:02 +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 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).

The following will provoke the crash:

__attribute__((noinline)) void somefunc(void) { }

int __clock_gettime(clockid_t clk, struct timespec *ts)
{
       ((int (*)(clockid_t, struct timespec *))vdso_func)(clk, ts);
       somefunc();
       return 88;
}

It seems like the VDSO is doing something to the stack.