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Date:   Mon, 19 Mar 2018 08:00:43 +0100
From:   Lukas Wunner <lukas@wunner.de>
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Laura Abbott <labbott@redhat.com>,
        Linus Walleij <linus.walleij@linaro.org>,
        Kees Cook <keescook@chromium.org>, linux-gpio@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-hardening@lists.openwall.com,
        Mathias Duckeck <m.duckeck@kunbus.de>,
        Nandor Han <nandor.han@ge.com>,
        Semi Malinen <semi.malinen@ge.com>,
        Patrice Chotard <patrice.chotard@st.com>
Subject: Re: [PATCH 1/4] gpio: Remove VLA from gpiolib
Message-ID: <20180319070043.GA25474@wunner.de>
References: <20180310001021.6437-1-labbott@redhat.com>
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 <20180317082509.GA2579@wunner.de>
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On Sun, Mar 18, 2018 at 09:34:12PM +0100, Rasmus Villemoes wrote:
> On 2018-03-18 15:23, Lukas Wunner wrote:
> >>> Other random thoughts: maybe two allocations for each loop iteration is
> >>> a bit much. Maybe do a first pass over the array and collect the maximal
> >>> chip->ngpio, do the memory allocation and freeing outside the loop (then
> >>> you'd of course need to preserve the memset() with appropriate length
> >>> computed). And maybe even just do one allocation, making bits point at
> >>> the second half.
> >>
> >> I think those are great ideas because the function is kind of a hotpath
> >> and usage of VLAs was motivated by the desire to make it fast.
> >>
> >> I'd go one step further and store the maximum ngpio of all registered
> >> chips in a global variable (and update it in gpiochip_add_data_with_key()),
> >> then allocate 2 * max_ngpio once before entering the loop (as you've
> >> suggested).  That would avoid the first pass to determine the maximum
> >> chip->ngpio.  In most systems max_ngpio will be < 64, so one or two
> >> unsigned longs depending on the arch's bitness.
> > 
> > Actually, scratch that.  If ngpio is usually smallish, we can just
> > allocate reasonably sized space for mask and bits on the stack,
> 
> Yes.
> 
> > and fall back to the kcalloc slowpath only if chip->ngpio exceeds
> > that limit.
> 
> Well, I'd suggest not adding that fallback code now, but simply add a
> check in gpiochip_add_data_with_key to ensure ngpio is sane (and refuse
> to register the chip otherwise), at least if we know that every
> currently supported/known chip is covered by the 256 (?).

The number 256 was arbitrarily chosen.  I really wouldn't be surprised
if gpiochips with more pins exist, but they're probably rare, so using
the slowpath seems fine, but dropping support for them completely would
be a regression.

E.g. many serially attached chips such as MAX3191X are daisy-chainable
and the driver deliberately doesn't impose an upper limit on the number
of chips because the spec doesn't contain one either.  To the OS a
daisy-chain of such chips appears as a single gpiochip with many pins.

Thanks,

Lukas