[PATCH v2] elf: Add optimization barrier for __ehdr_start and _end

[H.J. Lu]

On Mon, Jun 16, 2025 at 6:32 AM Sam James <sam@gentoo.org> wrote:
>
> "H.J. Lu" <hjl.tools@gmail.com> writes:
>
> > rtld.c has
> >
> > extern const ElfW(Ehdr) __ehdr_start attribute_hidden;
> > ...
> >   _dl_rtld_map.l_map_start = (ElfW(Addr)) &__ehdr_start;
> >
> > As
> >
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120653
> >
> > shows, compiler may generate run-time relocation on __ehdr_start with
> >
> > movq .LC0(%rip), %xmm0
> > ...
> > .section .data.rel.ro.local,"aw"
> > .align 8
> > .LC0:
> > .quad __ehdr_start
> >
> > This won't work before run-time relocation is finished in rtld.c.  Define
> > _dl_ehdr_start_before_reloc to allow x86-64 to generate LEA to avoid
> > run-time relocation on __ehdr_start in rtld.c.
>
> LGTM, but please give Florian a chance to object. Thanks for adding a
> test too.
>
> I still feel a bit uneasy about the general approach here
> (whack-a-mole). Florian had some thoughts at

That is the nature of ld.so.

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120653#c15 on doing better
> but I don't think that should block this simple (and backportable in due
> course) fix.
>
> I see Jakub has a broader suggestion as well on the bug.

Here is the v2 patch to add

diff --git a/elf/rtld.c b/elf/rtld.c
index 8e8f0e6253..e6a181dc31 100644
--- a/elf/rtld.c
+++ b/elf/rtld.c
@@ -477,7 +477,10 @@ _dl_start_final (void *arg, struct dl_start_final_info *inf
o)
   _dl_setup_hash (&_dl_rtld_map);
   _dl_rtld_map.l_real = &_dl_rtld_map;
   _dl_rtld_map.l_map_start = (ElfW(Addr)) &__ehdr_start;
+  /* Prevent run-time relocations against __ehdr_start and _end.  */
+  asm ("" : "+g" (_dl_rtld_map.l_map_start));
   _dl_rtld_map.l_map_end = (ElfW(Addr)) _end;
+  asm ("" : "+g" (_dl_rtld_map.l_map_end));
   /* Copy the TLS related data if necessary.  */
 #ifndef DONT_USE_BOOTSTRAP_MAP
 # if NO_TLS_OFFSET != 0

as Jakub suggested.

-- 
H.J.
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