linux-kernel - Re: [PATCH 3/4] zsmalloc use zs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4FABF9D4.8080303@vflare.org>
Date:	Thu, 10 May 2012 13:24:36 -0400
From:	Nitin Gupta <ngupta@...are.org>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/4] zsmalloc use zs_handle instead of void *

On 5/10/12 12:44 PM, Greg Kroah-Hartman wrote:
> On Thu, May 10, 2012 at 12:29:41PM -0400, Nitin Gupta wrote:
>> On 5/10/12 11:19 AM, Greg Kroah-Hartman wrote:
>>> On Thu, May 10, 2012 at 10:11:27AM -0500, Seth Jennings wrote:
>>>> On 05/10/2012 09:47 AM, Nitin Gupta wrote:
>>>>
>>>>> On 5/10/12 10:02 AM, Konrad Rzeszutek Wilk wrote:
>>>>>> struct zs {
>>>>>>      void *ptr;
>>>>>> };
>>>>>>
>>>>>> And pass that structure around?
>>>>>>
>>>>>
>>>>> A minor problem is that we store this handle value in a radix tree node.
>>>>> If we wrap it as a struct, then we will not be able to store it directly
>>>>> in the node -- the node will have to point to a 'struct zs'. This will
>>>>> unnecessarily waste sizeof(void *) for every object stored.
>>>>
>>>>
>>>> I don't think so. You can use the fact that for a struct zs var,&var
>>>> and&var->ptr are the same.
>>>>
>>>> For the structure above:
>>>>
>>>> void * zs_to_void(struct zs *p) { return p->ptr; }
>>>> struct zs * void_to_zs(void *p) { return (struct zs *)p; }
>>>
>>> Do like what the rest of the kernel does and pass around *ptr and use
>>> container_of to get 'struct zs'.  Yes, they resolve to the same pointer
>>> right now, but you shouldn't "expect" to to be the same.
>>>
>>>
>>
>> I think we can just use unsigned long as zs handle type since all we
>> have to do is tell the user that the returned value is not a
>> pointer. This will be less pretty than a typedef but still better
>> than a single entry struct + container_of stuff.
>
> But then you are casting the thing all around just as much as you were
> with the void *, right?
>
> Making this a "real" structure ensures type safety and lets the compiler
> find the problems you accidentally create at times :)
>

If we return a 'struct zs' from zs_malloc then I cannot see how we are 
solving the original problem of storing the handle directly in a radix 
node. If we pass a struct zs we will require pointing radix node to this 
struct, wasting sizeof(void *) for every object.   If we pass unsigned 
long, then this problem is solved and it also makes it clear that the 
passed value is not a pointer.

Its true that making it a real struct would prevent accidental casts to 
void * but due to the above problem, I think we have to stick with 
unsigned long.

Thanks,
Nitin

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/