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

[Florian Weimer <fweimer at redhat dot com>]

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