[PATCH v2] elf: Avoid nested functions in the loader (all ports) [BZ #27220]

[Fāng-ruì Sòng]

On Thu, Sep 23, 2021 at 10:13 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * Fāng-ruì Sòng:
>
> > For elf/dl-reloc.os on x86-64, I think the nested function version can
> > use *(rbp - offset) to get some arguments.
> > The new version needs more instructions but the cost is small compared
> > with the expensive _dl_lookup_symbol_x.
>
> I have a patch that reduces the number of _dl_lookup_symbol_x calls for
> the self-relocation (bootstrap or not):
>
>   [v3,3/3] elf: Rework exception handling in the dynamic loader [BZ #25486]
>   <https://patchwork.sourceware.org/project/glibc/patch/05e1b5b43416eba58f2bab968320fa3d38e33479.1607085588.git.fweimer@redhat.com/>
>
> It needs rebase on the current tree.
>
> We recently lost the internal _r_debug reference, which is probably a
> bug.  Other architectures have some other crud in ld.so.  But I think we
> can get the number of symbols used in self-relocation down to zero on
> all architectures fairly easily.
>
> Once we do that, how much of the bootstrap relocation goo is really
> left?
>
> Thanks,
> Florian
>

If self-relocations means relocations used in rtld.c and
elf/dl-reloc-static-pie.o:

The new version doesn't cause differences near _dl_lookup_symbol_x call sites.
I think the relocation code sequence doesn't change.

@@ -5757,7 +5789,7 @@
                        push    0
                        push    1
                        call     <L509>
-               0000000000004ec7:  R_X86_64_PLT32       _dl_lookup_symbol_x-0x4
+               0000000000004f27:  R_X86_64_PLT32       _dl_lookup_symbol_x-0x4
 <L509>:
                        pop     r8
                        pop     r9

The new rtld.os is slightly larger, but I guess that it is just
because GCC inlines one more place.