linux-kernel - Re: [PATCH V2] MMC: core: cap MMC card timeouts at 2 seconds.

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Date:	Tue, 29 May 2012 03:32:12 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	"Torne (Richard Coles)" <torne@...gle.com>
Cc:	cjb@...top.org, linus.walleij@...aro.org, jh80.chung@...sung.com,
	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] MMC: core: cap MMC card timeouts at 2 seconds.

On Mon, 2012-05-28 at 18:31 +0100, Torne (Richard Coles) wrote:
> From: "Torne (Richard Coles)" <torne@...gle.com>
> 
> MMC CSD info can specify very large, ridiculous timeouts, big enough to
> overflow timeout_ns on 32-bit machines. This can result in the card
> timing out on every operation because the wrapped timeout value is far
> too small.
> 
> Fix the overflow by capping the result at 2 seconds.  Cards specifying
> longer timeouts are almost certainly insane, and host controllers
> generally cannot support timeouts that long in any case.
> 
> 2 seconds should be plenty of time for any card to actually function;
> the timeout calculation code is already using 1 second as a "worst case"
> timeout for cards running in SPI mode.

Needs a 'Signed-off-by'.

> ---
>  drivers/mmc/core/core.c |   11 ++++++++++-
>  1 files changed, 10 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/mmc/core/core.c b/drivers/mmc/core/core.c
> index 0b6141d..3b4a9fc 100644
> --- a/drivers/mmc/core/core.c
> +++ b/drivers/mmc/core/core.c
> @@ -512,7 +512,16 @@ void mmc_set_data_timeout(struct mmc_data *data, const struct mmc_card *card)
>  	if (data->flags & MMC_DATA_WRITE)
>  		mult <<= card->csd.r2w_factor;
>  
> -	data->timeout_ns = card->csd.tacc_ns * mult;
> +	/*
> +	 * The timeout in nanoseconds may overflow with some cards. Cap it at
> +	 * two seconds both to avoid the overflow and also because host
> +	 * controllers cannot generally generate timeouts that long anyway.
> +	 */
> +	if (card->csd.tacc_ns <= (2 * NSEC_PER_SEC) / mult)
> +		data->timeout_ns = card->csd.tacc_ns * mult;
> +	else
> +		data->timeout_ns = 2 * NSEC_PER_SEC;

We clearly need to guard against overflow here, and this is the correct
way to clamp the multiplication.  I can't speak as to whether 2 seconds
is the right limit.

Ben.

>  	data->timeout_clks = card->csd.tacc_clks * mult;
>  
>  	/*

-- 
Ben Hutchings
Teamwork is essential - it allows you to blame someone else.

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