netdev - Re: [PATCH v3] net: bluetooth: hci_sock: Use 'const u32 *' instead of 'void *' for 2nd parameter of hci_test

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Date:	Sun, 8 Feb 2015 12:17:14 -0800
From:	Marcel Holtmann <marcel@...tmann.org>
To:	Chen Gang S <gang.chen@...rus.com.cn>
Cc:	Joe Perches <joe@...ches.com>,
	David Laight <David.Laight@...LAB.COM>,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	"Gustavo F. Padovan" <gustavo@...ovan.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	"linux-bluetooth@...r.kernel.org" <linux-bluetooth@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v3] net: bluetooth: hci_sock: Use 'const u32 *' instead of 'void *' for 2nd parameter of hci_test_bit()

Hi Chen,

>>>> hci_test_bit() does not modify 2nd parameter, so it is better to let it
>>>> be constant, or may cause build warning. The related warning (with
>>>> allmodconfig under xtensa):
>>>> 
>>>>  net/bluetooth/hci_sock.c: In function 'hci_sock_sendmsg':
>>>>  net/bluetooth/hci_sock.c:955:8: warning: passing argument 2 of 'hci_test_bit' discards 'const' qualifier from pointer target type [-Wdiscarded-array-qualifiers]
>>>>          &hci_sec_filter.ocf_mask[ogf])) &&
>>>>          ^
>>>>  net/bluetooth/hci_sock.c:49:19: note: expected 'void *' but argument is of type 'const __u32 (*)[4] {aka const unsigned int (*)[4]}'
>>>>   static inline int hci_test_bit(int nr, void *addr)
>>>>                     ^
>>>> 
>>>> hci_test_bit() always treats 2nd parameter is u32, and all callers also
>>>> know about it, so 2nd parameter of hci_test_bit() need use 'const u32 *'
>>>> instead of 'void *'.
>>>> 
>>>> C language treats the array function parameter as a pointer, so the
>>>> caller need not use '&' for the 2 demotion array, or it reports warning:
>>>> 'const unsigned int (*)[4]' is different with 'const unsigned int *'.
>>> 
>>> I still think you are possibly papering over potential bugs
>>> on big-endian 64 bit systems.
>>> 
>>> unsigned long vs u32.
>>> 
>>> How are the bits actually set?
>>> 
>> 
>>> From current usage of event_mask, "(u32 *) f->event_mask" is only for
>> event_mask data storage, not for calculation (always as "u32 *" for
>> calculation).
>> 
>>  [root@...alhost linux-next]# grep -rn "\<event_mask\>" include/net/bluetooth net/bluetooth
>>  include/net/bluetooth/hci_sock.h:51:	unsigned long event_mask[2];
> 
> e.g. use "unsigned char event_mask[2 * sizeof(unsigned long)]" instead
> of "unsigned long event_mask[2]".
> 
> There is still no any issue within "hci_sock.c" (although I am not sure
> whether this modification may cause issues in another modules outside
> kernel).

what about writing a test case for userspace that ensures that things are working correctly. As I said before, we left it this way since it is part of the API.

Regards

Marcel

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