[PATCH v2 1/3] pwm: Add pwm_cansleep() as exported API to users
florian.vaussard at epfl.ch
Mon Jan 28 10:46:38 EST 2013
Le 28/01/2013 16:01, Russell King - ARM Linux a écrit :
> On Mon, Jan 28, 2013 at 10:36:07AM +0100, Florian Vaussard wrote:
>> Le 28/01/2013 09:45, Peter Ujfalusi a écrit :
>>> hi Thierry,
>>> On 01/26/2013 06:40 AM, Thierry Reding wrote:
>>>>> + return pwm->chip->can_sleep;
>>>> Would it make sense to check for NULL pointers here? I guess that
>>>> passing NULL into the function could be considered a programming error
>>>> and an oops would be okay, but in that case there's no point in making
>>>> the function return an int. Also see my next comment.
>>> While it is unlikely to happen it is better to be safe, something like this
>>> will do:
>>> return pwm ? pwm->chip->can_sleep : 0;
>> Ok. And what about:
>> BUG_ON(pwm == NULL);
>> return pwm->chip->can_sleep;
> Let's get something straight.
> 1. Don't use BUG_ON() as some kind of willy nilly assert() replacement.
> Linus refused to have assert() in the kernel because assert() gets not
> only over-used, but also gets inappropriately used too.
> _Only_ _ever_ use BUG_ON() if continuing is going to cause user
> noticable data loss which is not reportable to userspace. In other
> words, block device queue corruption or the like - where bringing the
> system down is going to _save_ the system from itself.
> Otherwise, return an error and/or use WARN_ON().
> 2. If you want a slow kernel, then by all means check your arguments to
> your functions. While you're at it, why not check that strings which
> are passed contain only the characters you expect them to? And, if
> you're bothering to check against a NULL pointer, what about NULL+1
> pointers which are also invalid? Why not invent some function to
> ensure that the pointer is a valid kernel pointer. Maybe you'll have
> to interate the vmalloc lists too - yay, more code to be executed!
> That must be good!
> In your example, if you're going to check that pwm is non-NULL, what
> if pwm->chip is non-NULL? How far do you take this?
> Or... just like most of the core kernel does, it does _not_ verify on
> function entry that the pointer is "correct" unless it is explicitly
> defined that the function may take a NULL pointer (like kfree()).
> Everything else just goes right on and does the dereference - and if
> the pointer was wrong, we hope that the MMU faults and we get a kernel
> Have a read through the code in fs/ or kernel/ and see how many functions
> you can spot in there which validate their pointers which aren't dealing
> with data from userland.
> You'll find almost no function checking that an inode pointer is not NULL.
> Or a struct file pointer. Or a struct path pointer... etc.
> Yet, you come to ARM code, and it seems "popular" that pointer arguments
> need to be verified on every single function call. Why is this?
> I don't know if Andrew would like to inject something here (I've added
> him) on this subject...
The v3 does not contain the check.
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