[PATCH] ld: Add -z start-stop-gc to let __start_/__stop_ not retain C identifier name sections

[Fangrui Song]

On 2021-03-01, Alan Modra wrote:
>On Sat, Feb 27, 2021 at 09:03:19PM -0800, Fangrui Song wrote:
>> Unfortunately if there is an undefined weak "__start_xx" and no xx is retained =>
>> undefined symbol error due to ldlang.c:undef_start_stop .
>>
>> I don't know how to distinguish undefined strong and undefined weak in undef_start_stop.
>
>OK, so let's fix that first.
>
>If a weak reference to a __start_foo or __stop_foo symbol ends up
>having no definition due to all the foo sections being removed for
>some reason, undef_start_stop currently makes the symbol strong
>undefined.  That risks a linker undefined symbol error.  Fix that by
>making the symbol undefweak and also undo some dynamic symbol state.
>
>Note that saving the state of the symbol type at the time
>lang_init_start_stop runs is not sufficient.  The linker may have
>merged in a shared library reference by that point and made what was
>an undefweak in regular objects, a strong undefined.  So it is
>necessary to look at the ELF symbol flags to decide whether an
>undefweak is the proper resolution.
>
>Something probably should be done for COFF/PE too, but I'm unsure how
>to do go about that.
>
>	* ldlang.c (undef_start_stop): For ELF make undefined start/stop
>	symbols undefweak if that was how they were referenced.  Undo
>	dynamic state too.
>
>diff --git a/ld/ldlang.c b/ld/ldlang.c
>index 5ffc8444c7..a77e8fabef 100644
>--- a/ld/ldlang.c
>+++ b/ld/ldlang.c
>@@ -6808,6 +6808,19 @@ undef_start_stop (struct bfd_link_hash_entry *h)
> 	}
>       h->type = bfd_link_hash_undefined;
>       h->u.undef.abfd = NULL;
>+      if (bfd_get_flavour (link_info.output_bfd) == bfd_target_elf_flavour)
>+	{
>+	  const struct elf_backend_data *bed;
>+	  struct elf_link_hash_entry *eh = (struct elf_link_hash_entry *) h;
>+	  unsigned int was_forced = eh->forced_local;
>+
>+	  bed = get_elf_backend_data (link_info.output_bfd);
>+	  (*bed->elf_backend_hide_symbol) (&link_info, eh, TRUE);
>+	  if (!eh->ref_regular_nonweak)
>+	    h->type = bfd_link_hash_undefweak;
>+	  eh->def_regular = 0;
>+	  eh->forced_local = was_forced;
>+	}
>     }
> }

Thanks for taking care of the feature:)


Here is another interesting test.

.weak __start_xx
.weak __stop_xx

.global _start
_start:
   movq __start_xx@gotpcrel(%rip), %rdi
   movq __stop_xx@gotpcrel(%rip), %rsi

.section xx,"a",unique,0
.byte 0

.section xx,"a",unique,1
.byte 1

.section xx,"a",unique,2
.byte 2


ld-new a.o --gc-sections --print-gc-sections -pie -z start-stop-gc can discard 3 sections.
ld-new a.o --gc-sections --print-gc-sections -shared -z start-stop-gc can only discard 2 - there may be a lurking bug somewhere.