linux-kernel - Re: Need proper type casting before assignment, Remove compilation Warning.

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Message-ID: <57801F5B.20502@gmail.com>
Date:	Sat, 9 Jul 2016 03:17:07 +0530
From:	arvind Yadav <arvind.yadav.cs@...il.com>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	scottwood@...escale.com, qiang.zhao@...escale.com,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: Need proper type casting before assignment, Remove compilation
 Warning.

As per your concern, I have changed and submitted one more patch.

This answer of your all questions,
-Return type of 'qe_muram_alloc' is 'unsigned long', That Was trying to
assigned in ucc_fast_tx_virtual_fifo_base_offset and
ucc_fast_rx_virtual_fifo_base_offset. It will work on 32-bit architectures
But data can be loss on 64-bit architectures if 'qe_muram_alloc' will return
greater then MAX value of 'unsigned int'.


-Passing value in IS_ERR_VALUE() is wrong, as they pass an 'unsigned int'
into a function, It will through this compilation warning.
"
  include/linux/err.h:21:49: warning: cast to pointer from integer of 
different size [-Wint-to-pointer-cast]
  #define IS_ERR_VALUE(x) unlikely((unsigned long)(void *)(x) >= 
(unsigned long)-MAX_ERRNO)
                                                  ^
  include/linux/compiler.h:170:42: note: in definition of macro ‘unlikely’
  # define unlikely(x) __builtin_expect(!!(x), 0)
"

-Most users of IS_ERR_VALUE() in the kernel are wrong, as they
pass an 'unsigned int' into a function that takes an 'unsigned long'
argument. This happens to work because the type is sign-extended
on 64-bit architectures before it gets converted into an
unsigned type.

However, anything that passes an 'unsigned short' or 'unsigned int'
argument into IS_ERR_VALUE() is guaranteed to be broken, as are
8-bit integers and types that are wider than 'unsigned long'.


Thanks,
Arvind Yadav

On Friday 08 July 2016 09:03 PM, Guenter Roeck wrote:
> On Thu, Jul 07, 2016 at 10:31:11PM +0530, Arvind Yadav wrote:
>> -Return type of 'qe_muram_alloc' is 'unsigned long', That Was trying to
>> assigned in ucc_fast_tx_virtual_fifo_base_offset and
>> ucc_fast_rx_virtual_fifo_base_offset. These variable are 'unsigned int'.
>> So before assginment need a proper type casting.
> Are they ? In the upstream kernel, they seem to be "u32".
> -Yes, I have changed as per you suggestion.
>> -Passing value in IS_ERR_VALUE() is wrong, as they pass an 'int'
>> into a function that takes an 'unsigned long' argument.This happens
>> to work because the type is sign-extended on 64-bit architectures
>> before it gets converted into an unsigned type.
>>
> Not really sure I understand if/how this applies to the patch in question.
> I don't see an int passed to IS_ERR_VALUE(), I only see u32.
>
>> -Passing an 'unsigned short' or 'unsigned int'argument into
>> IS_ERR_VALUE() is guaranteed to be broken, as are 8-bit integers
>> and types that are wider than 'unsigned long'.
>>
> What does this have to do with this patch ?
>
>> -Any user will get compilation warning for that do not pass an
>> unsigned long' argument.
>>
> Sure, but that doesn't mean that typecasting the parameter to unsigned long
> does any good (other than hiding the real bug).
>
> Your subject line still does not list the affected subsystem and/or driver.
> Documentation/SubmittingPatches might give some hints about proper subject
> lines, and looking at other patches applied to the same file(s) might help
> as well.
>
> Also, if you want someone to review your patches, it helps to Cc: that
> someone.
>
>> Signed-off-by: Arvind Yadav <arvind.yadav.cs@...il.com>
>> ---
>>   drivers/soc/fsl/qe/ucc_fast.c | 11 +++++++----
>>   1 file changed, 7 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/soc/fsl/qe/ucc_fast.c b/drivers/soc/fsl/qe/ucc_fast.c
>> index a768931..98eed25 100644
>> --- a/drivers/soc/fsl/qe/ucc_fast.c
>> +++ b/drivers/soc/fsl/qe/ucc_fast.c
>> @@ -267,8 +267,10 @@ int ucc_fast_init(struct ucc_fast_info * uf_info, struct ucc_fast_private ** ucc
>>   
>>   	/* Allocate memory for Tx Virtual Fifo */
>>   	uccf->ucc_fast_tx_virtual_fifo_base_offset =
>> -	    qe_muram_alloc(uf_info->utfs, UCC_FAST_VIRT_FIFO_REGS_ALIGNMENT);
>> -	if (IS_ERR_VALUE(uccf->ucc_fast_tx_virtual_fifo_base_offset)) {
>> +		(unsigned int)qe_muram_alloc(uf_info->utfs,
> I don't see the point of this typecast.
>
>> +				UCC_FAST_VIRT_FIFO_REGS_ALIGNMENT);
>> +	if (IS_ERR_VALUE(
>> +		(unsigned long)uccf->ucc_fast_tx_virtual_fifo_base_offset)) {
> If sizeof(u32) == sizeof(unsigned long), this patch does not have an effect.
> If sizeof(u32) < sizeof(unsigned long), it does not change anything, and the
> resulting code is as wrong as it was before.
>
>>   		printk(KERN_ERR "%s: cannot allocate MURAM for TX FIFO\n",
>>   			__func__);
>>   		uccf->ucc_fast_tx_virtual_fifo_base_offset = 0;
>> @@ -278,10 +280,11 @@ int ucc_fast_init(struct ucc_fast_info * uf_info, struct ucc_fast_private ** ucc
>>   
>>   	/* Allocate memory for Rx Virtual Fifo */
>>   	uccf->ucc_fast_rx_virtual_fifo_base_offset =
>> -		qe_muram_alloc(uf_info->urfs +
>> +		(unsigned int)qe_muram_alloc(uf_info->urfs +
>>   			   UCC_FAST_RECEIVE_VIRTUAL_FIFO_SIZE_FUDGE_FACTOR,
>>   			   UCC_FAST_VIRT_FIFO_REGS_ALIGNMENT);
>> -	if (IS_ERR_VALUE(uccf->ucc_fast_rx_virtual_fifo_base_offset)) {
>> +	if (IS_ERR_VALUE(
>> +		(unsigned long)uccf->ucc_fast_rx_virtual_fifo_base_offset)) {
>>   		printk(KERN_ERR "%s: cannot allocate MURAM for RX FIFO\n",
>>   			__func__);
>>   		uccf->ucc_fast_rx_virtual_fifo_base_offset = 0;