[PATCH 2/3] elf: Replace a --defsym trick with an object file to be compatible with lld

H.J. Lu hjl.tools@gmail.com
Tue Jan 19 00:03:22 GMT 2021


On Mon, Dec 28, 2020 at 11:50 AM Fangrui Song via Libc-alpha
<libc-alpha@sourceware.org> 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.
>

This is irrelevant since  --defsym was used to create a temporary file which was
removed immediately after it was created:

/usr/bin/gcc   -nostdlib -nostartfiles -r -o
/export/users/hjl/build/gnu/tools-build/glibc-gitlab/build-x86_64-linux/elf/librtld.map.o
-Wl,--defsym=calloc=0 -Wl,--defsym=free=0 -Wl,--defsym=malloc=0
-Wl,--defsym=realloc=0 -Wl,--defsym=__stack_chk_fail=0
-Wl,--defsym=__stack_chk_fail_local=0 \
        '-Wl,-('
/export/users/hjl/build/gnu/tools-build/glibc-gitlab/build-x86_64-linux/elf/dl-allobjs.os
/export/users/hjl/build/gnu/tools-build/glibc-gitlab/build-x86_64-linux/libc_pic.a
-lgcc '-Wl,-)' -Wl,-Map,/export/users/hjl/build/gnu/tools-build/glibc-gitlab/build-x86_64-linux/elf/librtld.mapT
rm -f /export/users/hjl/build/gnu/tools-build/glibc-gitlab/build-x86_64-linux/elf/librtld.map.o
mv -f /export/users/hjl/build/gnu/tools-build/glibc-gitlab/build-x86_64-linux/elf/librtld.mapT
/export/users/hjl/build/gnu/tools-build/glibc-gitlab/build-x86_64-linux/elf/librtld.map

> ---
>  elf/Makefile | 11 ++++-------
>  1 file changed, 4 insertions(+), 7 deletions(-)
>
> diff --git a/elf/Makefile b/elf/Makefile
> index 0b4d78c874..299bf24b49 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -524,10 +524,6 @@ 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.
> @@ -536,9 +532,10 @@ 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 -

Please generate

.globl symbol;
symbol = 0;

to make it closer to -Wl,--defsym=symbol=0.

> +       $(reloc-link) -o $@.o $@T.o '-Wl,-(' $^ -lgcc '-Wl,-)' -Wl,-Map,$@T
> +       rm -f %@T.o $@.o
>         mv -f $@T $@
>
>  # For lld, skip preceding addresses and values before matching the archive and the member.
> --
> 2.29.2.729.g45daf8777d-goog
>


-- 
H.J.


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