Re: [PATCH] S390: Support PLT and GOT references in check-localplt.

[Florian Weimer <fweimer at redhat dot com>]

On 08/23/2016 01:28 PM, Stefan Liebler wrote:
> On 08/18/2016 12:42 PM, Stefan Liebler wrote:
> > On 08/17/2016 03:48 PM, Florian Weimer wrote:
> > > On 08/09/2016 04:22 PM, Stefan Liebler wrote:
> > > > On 08/09/2016 03:53 PM, Florian Weimer wrote:
> > > > > On 08/09/2016 03:34 PM, Stefan Liebler wrote:
> > > > > > After the commits __libc_memalign is only called in
> > > > > > elf/dl-minimal.c in
> > > > > > malloc() function in ld.so and gcc -O2/-O3 leads to R_390_GLOB_DAT
> > > > > > instead of R_390_JMP_SLOT. __libc_memalign is called via
> > > > > > function-pointer loaded from GOT instead of calling via a plt-stub.
> > > > >
> > > > > Huh.  I wonder if this is intended.  Surely the malloc
> > > > > implementation in
> > > > > elf/dl-minimal.c should always call the __libc_memalign implementation
> > > > > in the same file?
> > > > >
> > > > > Thanks,
> > > > > Florian
> > > >
> > > > Please have a look at the other localplt.data files, e.g.
> > > > sysdeps/x86_64/localplt.data:
> > > > # The dynamic loader uses __libc_memalign internally to allocate
> > > > aligned
> > > > # TLS storage. The other malloc family of functions are expected to
> > > > allow
> > > > # user symbol interposition.
> > > > ld.so: __libc_memalign + RELA R_X86_64_GLOB_DAT
> > > >
> > > > All contains the __libc_memalign relocation.
> > > > On s390x it has also existed before but a plt-stub was used. If you now
> > > > use the compiler flag -fno-optimize-sibling-calls you'll get the
> > > > plt-stub as before. With this optimization the relocation is done via a
> > > > function pointer.
> > >
> > > Okay, but shouldn't be __libc_memalign in dl-minimal.c declared as
> > > attribute_hidden?
> > >
> > > Thanks,
> > > Florian
> > I don't know. Can you simply do this as __libc_memalign is also
> > mentioned in elf/Versions file?
> > ld {
> >   GLIBC_2.0 {
> >     # Function from libc.so which must be shared with libc.
> >     __libc_memalign; calloc; free; malloc; realloc;
> >
> >     _r_debug;
> >   }
>
> Is there any news about __libc_memalign?
> Otherwise I'd like to commit this patch.
> I think if you change something about __libc_memalign you have to touch
> the other architectures as well.

Sorry, I don't feel qualified to comment on this.

What has changes is that we no longer reference __libc_memalign, in 
order to simplify the interposition of a custom malloc.

I find all this rather puzzling because we really do not want to 
interpose the dl-minimal.c malloc for the whole process, and yet the 
elf/Versions file lists it …

Florian