linux-kernel - Re: [PATCH] gpio: ep93xx: fix test for end of loop

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Date:   Thu, 6 Sep 2018 16:18:44 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Linus Walleij <linus.walleij@...aro.org>,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] gpio: ep93xx: fix test for end of loop

On 06/09/18 15:48, Dan Carpenter wrote:
> On Thu, Sep 06, 2018 at 02:50:44PM +0100, Colin Ian King wrote:
>> On 06/09/18 14:33, Dan Carpenter wrote:
>>> The problem is that if port == ARRAY_SIZE() and "gc == &epg->gc[port]"
>>> then that should be treated as invalid.
>>>
>>> Fixes: fd935fc421e7 ("gpio: ep93xx: Do not pingpong irq numbers")
>>> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
>>>
>>> diff --git a/drivers/gpio/gpio-ep93xx.c b/drivers/gpio/gpio-ep93xx.c
>>> index 68a416fc3141..b0699f57ddf5 100644
>>> --- a/drivers/gpio/gpio-ep93xx.c
>>> +++ b/drivers/gpio/gpio-ep93xx.c
>>> @@ -80,7 +80,7 @@ static int ep93xx_gpio_port(struct gpio_chip *gc)
>>>  		port++;
>>>  
>>>  	/* This should not happen but is there as a last safeguard */
>>> -	if (gc != &epg->gc[port]) {
>>> +	if (port == ARRAY_SIZE(epg->gc)) {
>>>  		pr_crit("can't find the GPIO port\n");
>>>  		return 0;
>>>  	}
>>>
>>
>> Good catch! I overlooked that one.
> 
> It's unfortunate but I don't think any of our static checkers would have
> caught it.  You found this bug because of cppcheck, right?  I know it
> warns about the bounds test after use.  Does it also warn about the out
> of bounds?

It can catch these, not sure how well it compares to the other tools I use.

> 
> Smatch doesn't warn about the out of bounds read because Smatch has bad
> handling of loops.  Smatch is supposed to warn about the test after use
> but that code is buggy.  I will investigate what's going on.
> 
> I have an unpublished test so Smatch does warn that the port < sizeof()
> means that port is in terms of bytes but we're using it as an array
> offset.  That's how I noticed this code, but it only warns about the
> first use, so it warns about the loop but not the post-loop test.
> 
> I should fix Smatch's handling of loops so that we know this function
> originally could return 0-8 and you maybe get a warning in the caller.
> Unfortunately, I'm not sure I would have paid very much attention to a
> warning like that because they tend to be prone to false positives.  We
> have a lot of loops that do:
> 
> 	for (i = 0; i < ARRAY_SIZE(array); i++) {
> 		if (foo <= array[i])
> 			break;
> 	}

..yep, the are a lot of these to fix up for sure.

> 
> And the last element of the array[] is UINT_MAX so we always break.
> It's a lot of work but not necessarily difficult work.
> 
> regards,
> dan carpenter
>