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

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Date:	Fri, 1 Jun 2012 10:32:40 +0100
From:	"Torne (Richard Coles)" <torne@...gle.com>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Ben Hutchings <ben@...adent.org.uk>, 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 1 June 2012 10:31, Torne (Richard Coles) <torne@...gle.com> wrote:
> On 1 June 2012 09:35, Adrian Hunter <adrian.hunter@...el.com> wrote:
>> On 29/05/12 05:32, Ben Hutchings wrote:
>>> 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.
>>
>> The host controllers I have looked at have a limit of around 2.5 seconds.
>>
>> But why not just use the size of the type as the limit? e.g.
>>
>>        if (card->csd.tacc_ns <= UINT_MAX / mult)
>>                data->timeout_ns = card->csd.tacc_ns * mult;
>>        else
>>                data->timeout_ns = UINT_MAX;
>
> The host controller drivers don't seem to all do a very good job of
> preventing further overflows or handling large values correctly
> (though some do). sdhci takes the especially annoying additional step
> of printk'ing a warning for *every single MMC command* where
> data->timeout_ns is larger than the controller can accommodate.
> Capping it to a value with a sensible order of magnitude seems to make
> it more likely that cards with obviously bogus CSD parameters will
> actually work. I don't object to using a larger number for the limit,
> but UINT_MAX on a 64-bit system obviously doesn't limit this at all
> and will leave you with timeouts up to 17 minutes, which seems
> ridiculous :)

Er, not 17 minutes; 102.4 seconds as I used later in my mail. SD cards
have their timeouts capped already, so their larger 100x multiplier is
not a problem; 102.4 seconds is the longest for an MMC card.

> My original motivation for this patch is that I have a device with an
> eMMC that specifies a 25.5 second timeout, attached to a sdhci host
> whose maximum timeout is 2.8 seconds. Originally I proposed a patch to
> just remove the warning in sdhci, but nobody replied, and when I
> realised there was actually an overflow happening I opted to fix that
> instead.
>
> So, yeah, we could use UINT_MAX, but then at minimum I also need to
> kill the warning in sdhci to make my device work, and probably all the
> host controller drivers need to be checked to make sure they don't use
> timeout_ns in a way that can overflow.
>
> I've also just noticed that struct mmc_data's comment for timeout_ns
> says /* data timeout (in ns, max 80ms) */ which is not true (the max
> is 102.4 seconds if my math is correct), which may have contributed to
> the host drivers not being too careful :)
>
> What do you think?
>
>>>
>>> Ben.
>>>
>>>>      data->timeout_clks = card->csd.tacc_clks * mult;
>>>>
>>>>      /*
>>>
>>
>
>
>
> --
> Torne (Richard Coles)
> torne@...gle.com



-- 
Torne (Richard Coles)
torne@...gle.com
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