Re: [Linaro-mm-sig] [PATCHv3 2/2] staging: android: ion: Add ioctl to query available heaps

[Laura Abbott <labbott@redhat.com>]

On 09/07/2016 12:37 PM, Arnd Bergmann wrote:
> On Wednesday, September 7, 2016 11:49:59 AM CEST Laura Abbott wrote:
>
>> -	if (dir & _IOC_WRITE)
>> -		if (copy_from_user(&data, (void __user *)arg, _IOC_SIZE(cmd)))
>> -			return -EFAULT;
>> +	/*
>> +	 * The copy_from_user is unconditional here for both read and write
>> +	 * to do the validate. If there is no write for the ioctl, the
>> +	 * buffer is cleared
>> +	 */
>> +	if (copy_from_user(&data, (void __user *)arg, _IOC_SIZE(cmd)))
>> +		return -EFAULT;
>> +
>> +	ret = validate_ioctl_arg(cmd, &data);
>> +	if (WARN_ON_ONCE(ret))
>> +		return ret;
>
> I noticed that the WARN_ON_ONCE warns about invalid user input,
> but I think we tend to normally just use WARN_ON for things that
> go wrong inside of the kernel or in hardware.
>
> Maybe better use printk_once() or printk_ratelimited.
>

Sure, the error code should hopefully be enough of a hint to
userspace to maybe check the log.

> Is there any noticeable overhead in always copying the structure?
> copy_from_user() can be a bit slow depending on debugging or
> security features, and it seems unnecessary if the validation
> is only done for one of the commands.
>

Good point. It made sense with some of the other ioctls (specifically
the ABI) but isn't necessary now. We can evaluate later when other
ioctls get added.

> Otherwise the patch looks good to me.
>
> 	Arnd
>

Thanks!

Laura