[maemo-developers] N900 kernel 2.6.28 no compiles after changing cb[] array size

[From: Sergio Lembo]

Hi,

  Indeed that seems to be the problem.
  Thank you.

Regards,

Sergio

Quoting Nathan Anderson <nathan at andersonsplace.net>:

>  Sergio,
>
> 	Maybe the actual problem is that 62 is not a multiple of 4.   the
> sizeof(void (*)) should be 4 on 32 bit platforms.   So if you read that; you
> get:
>> 62-8 (=54)
>> 54 / 4 (=13.5)
>
> On the old value you get
>> 48-8 (=40)
>> 40/4 (=10)
>
>
> What happens if you change it to
> 	char                    cb[64];
>
> The compiler could be having a issue because of 13.5 and just masking the
> issue until later.  (Gotta love C/C++ compilers, you can chase your tail
> sometimes <G>)
>
> Nathan
>
>
>
>
>
> -----Original Message-----
> From: maemo-developers-bounces at maemo.org
> [mailto:maemo-developers-bounces at maemo.org] On Behalf Of Sergio Lembo
> Sent: Thursday, June 17, 2010 1:59 PM
> To: maemo-developers at maemo.org
> Subject: RE: N900 kernel 2.6.28 no compiles after changing cb[] array size
>
>
> Hi,
>
> Thank you for your answer.
>
> I observe that in include/net/mac80211.h the defines pasted below are
> introducing a non-linear sizing (observe the hardcoded number 8) that may
> cause problems for some ranges and thus validating (sizeof(struct
> ieee80211_tx_info) > sizeof(skb->cb)).
>
> #define IEEE80211_TX_INFO_DRIVER_DATA_SIZE \
> 	(sizeof(((struct sk_buff *)0)->cb) - 8)
>
> #define IEEE80211_TX_INFO_DRIVER_DATA_PTRS \
> 	(IEEE80211_TX_INFO_DRIVER_DATA_SIZE / sizeof(void *))
>
> Although this is not directly the problem that I have and
>
>    BUILD_BUG_ON(sizeof(struct ieee80211_tx_info) > sizeof(skb->cb));
>
> is failing for some other reason that I did not find yet..
>
> Regards,
>
> Sergio
>
>
>
>
> Quoting Nathan Anderson <nathan at andersonsplace.net>:
>
>> Sergio,
>>
>> 	If I had a guess, I would guess that the n900 probably has a
>> different set of compile time defines, that cause the ieee80211
>> structure to be defined differently on the n900 vs a stock machine.
>> The compile time failsafe is to make sure that the structure iee80211 is
> larger than skb->cb.
>> Run a grep on the code to find out where/how the ieee80211 structure
>> is being defined and you will probably find the issue.
>>
>> Nathan.
>>
>> -----Original Message-----
>> From: maemo-developers-bounces at maemo.org
>> [mailto:maemo-developers-bounces at maemo.org] On Behalf Of Sergio Lembo
>> Sent: Thursday, June 17, 2010 3:08 AM
>> To: maemo-developers at maemo.org
>> Subject: N900 kernel 2.6.28 no compiles after changing cb[] array size
>>
>>
>> Hi,
>>
>>    I am having an inconsistent state in the N900 kernel 2.6.28 when I
>> make a very simple
>> change:
>>
>>    The problem is reproduced easily by just increasing the size of the
>> control buffer cb[] in struct sk_buff{} (include/linux/skbuff.h):
>>
>> -       char                    cb[48];
>> -
>> +       char                    cb[62];
>>
>>    Using cb[62] I get a compilation error :
>>
>> CC [M]  net/mac80211/main.o
>> net/mac80211/main.c: In function 'ieee80211_init':
>> net/mac80211/main.c:1026: error: size of array 'type name' is negative
>> make[2]: *** [net/mac80211/main.o] Error 1
>> make[1]: *** [net/mac80211] Error 2
>> make: *** [net] Error 2
>>
>>    The line with the error is a failsafe at compilation time
>> (net/mac80211/main.c):
>>
>>         BUILD_BUG_ON(sizeof(struct ieee80211_tx_info) >
>> sizeof(skb->cb));
>>
>>    Bypassing such failsafe line the kernel compiles Ok, but the binary
>> flashed to the N900 fails to boot the system.
>>
>>    I observe that such compilation error does not happen when
>> compiling a plain 2.6.28 kernel for a PC.
>>
>>    And note that this error is just caused by changing the size of
>> cb[] from
>> 48 to 62; that is the only modification we made to the kernel sources.
>>
>>
>> Regards,
>>
>> Sergio
>>
>>
>>
>> _______________________________________________
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>>
>>
>
>
>
> --
> -----------------------
> Sergio Lembo
> sergio.lembo at tkk.fi
> -----------------------
>
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>
>



-- 
-----------------------
Sergio Lembo
sergio.lembo at tkk.fi
-----------------------