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From:   "Jason A. Donenfeld" <Jason@zx2c4.com>
Date:   Mon, 30 Dec 2019 16:10:02 +0100
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Subject: Re: [PATCH] mips: vdso: conditionalize 32-bit time functions on COMPAT_32BIT_TIME
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>,
        Paul Burton <paulburton@kernel.org>,
        Vincenzo Frascino <vincenzo.frascino@arm.com>,
        Christian Brauner <christian.brauner@canonical.com>
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On Mon, Dec 30, 2019 at 3:37 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> On Mon, Dec 30, 2019 at 1:34 PM Arnd Bergmann <arnd@arndb.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.