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Re: [PATCHi v2] aarch64: Add split-stack TCB field



On 14/03/2017 03:53, Florian Weimer wrote:
> On 03/14/2017 02:17 AM, Carlos O'Donell wrote:
>> On 02/14/2017 05:03 AM, Florian Weimer wrote:
>>> On 02/13/2017 08:45 PM, Adhemerval Zanella wrote:
>>>> +/* This is the size we need before TCB.  Check if there is room
>>>> for +   tcbprehead_t in struct pthread's final padding and if not
>>>> add it on +   required pre-tcb size.  */ +# define TLS_PRE_TCB_SIZE
>>>> \ +  (sizeof (struct pthread)                        \ +   +
>>>> (PTHREAD_STRUCT_END_PADDING < sizeof (tcbprehead_t)        \ +
>>>> ? ALIGN_UP (sizeof (tcbprehead_t), sizeof (struct pthread))    \ +
>>>> : 0))
>>>
>>> How does this preserve the alignment of struct pthread?
>>>
>>> It's also not clear to me how the “version control” aspect of
>>> __tcb_private_ss is supposed to work.  If the intent is to prevent
>>> loading of split-stack binaries with an older glibc, then a data
>>> symbol would be a safer choice.
>>
>> Why is a data symbol a safer choice?
> 
> No lazy binding and hence a more well-defined failure mode.
> 
> Thanks,
> Florian
> 

Carlos, I also see Florian suggestion a better approach.  Using a weak
function symbol, it will fail at runtime after symbol resolution with
an undefined jump to a null symbol (for aarch64 it will be a segfault
anyway), while with data symbol loader will explicit dump the missing
symbol.


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