[PATCH] Add private_function for private functions within glibc [BZ #21598]

[Florian Weimer]

On 08/08/2017 08:25 PM, H.J. Lu wrote:
> which is incompatible with Shadow Stack in Intel Control-flow Enforcement
> Technology (CET) instructions:
> 
> https://software.intel.com/sites/default/files/managed/4d/2a/control-flow-enforcement-technology-preview.pdf
> 
> since shadow stack doesn't match return stack.  We need to use register
> indirect branch via %ecx.  That means only 2 parameters can be passed
> in registers for external function calls with lazy binding when shadow
> stack is enabled.  However, internal_function, which should be used only
> with hidden function within libc.so, are also used with private function
> calls between different shared objects of glibc.

I think we should work towards removing internal_function instead.  It's
an unnecessary maintenance overhead for those working on other
architectures besides i386.

In addition, the GCC manual currently says this about %ecx:

“
Systems with the GNU C Library version 2.1 or higher and FreeBSD are
believed to be safe since the loaders there save EAX, EDX and ECX.
”
<https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html>

This means that you can recover %ecx for internal ld.so with some form
of ABI compatibility mark-up only because we pretty much guarantee that
regparm (3) is compatible with the GNU ABI.

Thanks,
Florian