linux-kernel - Re: [musl] Re: [alsa-devel] [PATCH v7 8/9] ALSA: add new 32-bit layout for snd_pcm_mmap

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Date:   Fri, 8 Oct 2021 13:45:45 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     musl@...ts.openwall.com
Cc:     Arnd Bergmann <arnd@...db.de>, Rich Felker <dalias@...c.org>,
        Michael Forney <mforney@...rney.org>,
        ALSA Development Mailing List <alsa-devel@...a-project.org>,
        Takashi Iwai <tiwai@...e.com>,
        Baolin Wang <baolin.wang@...aro.org>,
        y2038 Mailman List <y2038@...ts.linaro.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Mark Brown <broonie@...nel.org>,
        Baolin Wang <baolin.wang7@...il.com>
Subject: Re: [musl] Re: [alsa-devel] [PATCH v7 8/9] ALSA: add new 32-bit
 layout for snd_pcm_mmap_status/control

On Fri, Oct 8, 2021 at 1:11 PM Takashi Iwai <tiwai@...e.de> wrote:
> On Fri, 08 Oct 2021 11:24:39 +0200, Arnd Bergmann wrote:

> >
> > I've tried to understand this part of musl's convert_ioctl_struct(), but I just
> > can't figure out whether it does the conversion based the on the layout that
> > is currently used in the kernel, or based on the layout we should have been
> > using, and would use with the above fix. Rich, can you help me here?
>
> So, at this moment, I'm not sure whether we should correct the struct
> at all.  This will lead to yet more breakage, and basically the struct
> itself *works* -- the only bug is in 32bit compat handling in the
> kernel (again).

I'm still unsure if the musl fallback code is correct or not.

> The below is a revised kernel patch (again untested), just correcting
> the behavior of 32bit compat mode.  32bit apps on 32bit kernel work
> fine as is, as well as 64bit apps on 64bit kernel.

Right, this should cover all cases of the ioctl itself misbehaving.
In addition, we still need to disallow the mmap() interface on compat
kernels then. Strictly speaking, we could allow the snd_pcm_mmap_status
but not snd_pcm_mmap_control to be mapped, but I'm not sure if
that's better than disallowing both.

       Arnd