Re: [PATCH] Use libc_hidden_proto / _def for hidden wchar ifunc symbols.

[Stefan Liebler <stli at linux dot vnet dot ibm dot com>]

On 11/13/2017 06:30 PM, H.J. Lu wrote:
> On Mon, Nov 13, 2017 at 9:11 AM, Stefan Liebler <stli@linux.vnet.ibm.com> wrote:
> > On 11/13/2017 02:58 PM, H.J. Lu wrote:
> > > On Mon, Nov 13, 2017 at 3:55 AM, Florian Weimer <fweimer@redhat.com>
> > > wrote:
> > > > On 11/13/2017 11:16 AM, Stefan Liebler wrote:
> > > > > This patch adds the libc_hidden_proto / libc_hidden_def construct.
> > > > > Then the __GI_* symbols are the default-ifunc-variants which can be
> > > > > called
> > > > > without PLT.
> > > >
> > > >
> > > >
> > > > attribute_hidden and *_hidden_{proto,def} conflict on some architectures.
> > > > You need to remove attribute_hidden as part of this change.
> > >
> > >
> > > That is true.  On i686, a hidden IFUNC function inside libc.so must be
> > > compiled
> > > with -fPIC via PLT since EBX must be loaded with GOT first.   This
> > > isn't an issue
> > > for x86-64 since PLT uses PC-relative addressing.  In this case, we
> > > should remove
> > > hidden attribute, instead of using __GI_* symbols, if we sill want to use
> > > IFUNC
> > > inside libc.so.
> >
> > On s390/s390x, just removing attribute_hidden in wchar.h is fine, too. Then
> > r12 will be setup with the GOT-pointer and IFUNC will be used inside
> > libc.so.
> >
> > What was your original intention? Getting rid of "PLT" or "PLT and IFUNC"
> > for calls inside libc.so?
>
> The original intention is to remove PLT.  But it doesn't work for targets which
> need to set up a special register for PLT which is required by IFUNC.
But even on x86_64, __wcsnlen is called via a PLT-stub (with 
attribute_hidden). This indirection is removed with the __GI_* symbols.
But then we don't have the advantage of the IFUNC variants.

> > > Now I have question, is there a way to apply attribute_hidden to a
> > > function
> > > depending on architecture? For example, we remove attribute_hidden
> > > from __wcsnlen, __wcscat, __wcsncpy, __wcpncpy,  __wcschrnul in headers
> > > under include/.   For x86, we mark them hidden in a header file under
> > > sysdeps/x86?
> >
> > I don't know if duplicating the wchar.h file is such a good idea.
>
> Can we add
>
> #include <wcharP.h>
>
> to include/wchar.h and add a dummy sysdeps/generic/wcharP.h?  Then
> I can add sysdeps/x86_64/wcharP.h to hide __wcsnlen, __wcscat, __wcsncpy,
> _wcpncpy,  __wcschrnul
I think we should ask Wilco, Zack, ... if an architecture dependent 
wcharP.h is possible or not. They spent some effort to cleanup the 
string-headers.