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Date:   Fri, 18 Jan 2019 20:39:32 +0100
Message-ID: <s5hzhrxzrez.wl-tiwai@suse.de>
From:   Takashi Iwai <tiwai@suse.de>
To:     "Mark Brown" <broonie@kernel.org>
Cc:     "Jaroslav Kysela" <perex@perex.cz>, <alsa-devel@alsa-project.org>,
        <arnd@arndb.de>, <keescook@chromium.org>,
        <bgoswami@codeaurora.org>, <sr@denx.de>, <gustavo@embeddedor.com>,
        <philburk@google.com>, <willy@infradead.org>,
        <mchehab+samsung@kernel.org>, <sboyd@kernel.org>,
        <vkoul@kernel.org>, "Baolin Wang" <baolin.wang@linaro.org>,
        <daniel.thompson@linaro.org>, <leo.yan@linaro.org>,
        <mathieu.poirier@linaro.org>, <srinivas.kandagatla@linaro.org>,
        <anna-maria@linutronix.de>, <corbet@lwn.net>,
        <jmiller@neverware.com>, <ckeepax@opensource.wolfsonmicro.com>,
        <joe@perches.com>, <o-takashi@sakamocchi.jp>, <colyli@suse.de>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH] ALSA: core: Add DMA share buffer support
In-Reply-To: <20190118190805.GF6260@sirena.org.uk>
References: <290f6d3a5fe288b87480cc5fa12c5139728daeca.1547787189.git.baolin.wang@linaro.org>
        <81e894ba-acad-2fd4-996d-8d35edd8825a@perex.cz>
        <20190118190805.GF6260@sirena.org.uk>
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On Fri, 18 Jan 2019 20:08:05 +0100,
Mark Brown wrote:
> 
> On Fri, Jan 18, 2019 at 10:35:44AM +0100, Jaroslav Kysela wrote:
> 
> >   the tinyalsa implementation does not show much - it's equal to the
> > standard mmap access for the PCM devices. Even considering the Mark's
> > text, there must be an arbiter (sound server) which communicates with
> > the producer or consumer to control the data flow. I really would like
> > to see a real usage for this.
> 
> Right, the driving force behind implementing this is Android which had
> been using an out of tree version of this approach based on ION but
> that's run into trouble due to other outside changes.
> 
> >   It seems to me that the only point to implement this is the
> > permissions. We already have O_APPEND mode for the PCM file descriptor
> > which can reuse the PCM device multiple times (mmap the buffer to
> > multiple tasks). I would probably go in this way and add more extended
> > permission control for the PCM device, so permissions can be restricted
> > for the passed descriptor to the producer or the consumer task. In this
> > way, the restricted task might reuse other control mechanism offered buy
> > the PCM file descriptor without requesting the arbiter to do so (like
> > read the actual position in the DMA buffer, get the audio delay or so -
> > reduce context switches).
> 
> One concern I have with doing some ALSA-specific custom permissions
> thing is integration with frameworks like SELinux (they'd presumably
> need to learn about the ALSA specific stuff to manage it).  It also
> seems like it's adding a lot more security sensitive interfaces and 
> code which which will require audit and review, one of the things I 
> really like about this approach is that it's incredibly simple from
> the security point of view.

Well, I wonder what makes it more difficult by the approach Jaroslav
suggested.  With O_APPEND, you can just call mmap() normally, and
that's all. What's the merit of dma-buf approach wrt the security?

BTW, the suggested patch seems to have a problem when the attached PCM
performs hw_free.  Then the mapped target will be gone while another
process still mapping it.  And the code looks pretty racy.


thanks,

Takashi