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, 08 Oct 2021 13:53:15 +0200
From:   Takashi Iwai <tiwai@...e.de>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     musl@...ts.openwall.com, 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, 08 Oct 2021 13:45:45 +0200,
Arnd Bergmann wrote:
> 
> 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.

We need to verify the current behavior in anyway...

> > 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.

IIRC, the compat mmap is already disallowed even for the
SNDRV_PCM_MMAP_OFFSET_CONTROL_NEW (in pcm_control_mmap_allowed()), so
no need to change around that.


thanks,

Takashi