elf: Replace a --defsym trick with an object file to be compatible with lld

The existing code specifies -Wl,--defsym=malloc=0 and other malloc.os
definitions before libc_pic.a so that libc_pic.a(malloc.os) is not
fetched. This trick is used to avoid multiple definition errors which
would happen as a chain result:

  dl-allobjs.os has an undefined __libc_scratch_buffer_set_array_size
  __libc_scratch_buffer_set_array_size fetches libc_pic.a(scratch_buffer_set_array_size.os)
  libc_pic.a(scratch_buffer_set_array_size.os) has an undefined free
  free fetches libc_pic.a(malloc.os)
  libc_pic.a(malloc.os) has an undefined __libc_message
  __libc_message fetches libc_pic.a(libc_fatal.os)

  libc_fatal.os will cause a multiple definition error (__GI___libc_fatal)
  >>> defined at dl-fxstatat64.c
  >>>            /tmp/p/glibc/Release/elf/dl-allobjs.os:(__GI___libc_fatal)
  >>> defined at libc_fatal.c
  >>>            libc_fatal.os:(.text+0x240) in archive /tmp/p/glibc/Release/libc_pic.a

lld processes --defsym after all input files, so this trick does not
suppress multiple definition errors with lld. Split the step into two
and use an object file to make the intention more obvious and make lld
work.

This is conceptually more appropriate because --defsym defines a SHN_ABS
symbol while a normal definition is relative to the image base.

See https://sourceware.org/pipermail/libc-alpha/2020-March/111910.html
for discussions about the --defsym semantics.
---
 elf/Makefile | 11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

On Sat, Apr 11, 2020 at 3:16 PM Fangrui Song <maskray@google.com> wrote:
>
> The existing code specifies -Wl,--defsym=malloc=0 and other malloc.os
> definitions before libc_pic.a so that libc_pic.a(malloc.os) is not
> fetched. This trick is used to avoid multiple definition errors which
> would happen as a chain result:
>
>   dl-allobjs.os has an undefined __libc_scratch_buffer_set_array_size
>   __libc_scratch_buffer_set_array_size fetches libc_pic.a(scratch_buffer_set_array_size.os)
>   libc_pic.a(scratch_buffer_set_array_size.os) has an undefined free
>   free fetches libc_pic.a(malloc.os)
>   libc_pic.a(malloc.os) has an undefined __libc_message
>   __libc_message fetches libc_pic.a(libc_fatal.os)
>
>   libc_fatal.os will cause a multiple definition error (__GI___libc_fatal)
>   >>> defined at dl-fxstatat64.c
>   >>>            /tmp/p/glibc/Release/elf/dl-allobjs.os:(__GI___libc_fatal)
>   >>> defined at libc_fatal.c
>   >>>            libc_fatal.os:(.text+0x240) in archive /tmp/p/glibc/Release/libc_pic.a
>
> lld processes --defsym after all input files, so this trick does not
> suppress multiple definition errors with lld. Split the step into two
> and use an object file to make the intention more obvious and make lld
> work.
>
> This is conceptually more appropriate because --defsym defines a SHN_ABS
> symbol while a normal definition is relative to the image base.
>
> See https://sourceware.org/pipermail/libc-alpha/2020-March/111910.html
> for discussions about the --defsym semantics.
> ---
>  elf/Makefile | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/elf/Makefile b/elf/Makefile
> index 9b1d58c7ad..de5dff649d 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -497,10 +497,6 @@ rtld-stubbed-symbols = \
>    malloc \
>    realloc \
>
> -# The GCC arguments that implement $(rtld-stubbed-symbols).
> -rtld-stubbed-symbols-args = \
> -  $(patsubst %,-Wl$(comma)--defsym=%=0, $(rtld-stubbed-symbols))
> -
>  ifeq ($(have-ssp),yes)
>  # rtld is not built with the stack protector, so these references will
>  # go away in the rebuilds.
> @@ -509,9 +505,10 @@ endif
>
>  $(objpfx)librtld.map: $(objpfx)dl-allobjs.os $(common-objpfx)libc_pic.a
>         @-rm -f $@T
> -       $(reloc-link) -o $@.o $(rtld-stubbed-symbols-args) \
> -               '-Wl,-(' $^ -lgcc '-Wl,-)' -Wl,-Map,$@T
> -       rm -f $@.o
> +       echo '$(patsubst %,.globl %;,$(rtld-stubbed-symbols)) $(patsubst %,%:,$(rtld-stubbed-symbols))' | \
> +               $(CC) -o $@T.o $(ASFLAGS) -c -x assembler -
> +       $(reloc-link) -o $@.o $@T.o '-Wl,-(' $^ -lgcc '-Wl,-)' -Wl,-Map,$@T
> +       rm -f %@T.o $@.o
>         mv -f $@T $@
>
>  $(objpfx)librtld.mk: $(objpfx)librtld.map Makefile
> --
> 2.26.0.110.g2183baf09c-goog
>

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