[1/4] elf: Introduce the rtld-stubbed-symbols makefile variable

  This generalizes a mechanism used for stack-protector support, so
that it can be applied to other symbols if required.

Tested on i686-linux-gnu without the stack protector, and on
x86_64-linux-gnu with stack-protector-strong.
---
 elf/Makefile | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)
  

On 2/8/20 2:00 PM, Florian Weimer wrote:
> This generalizes a mechanism used for stack-protector support, so
> that it can be applied to other symbols if required.
> 
> Tested on i686-linux-gnu without the stack protector, and on
> x86_64-linux-gnu with stack-protector-strong.

OK for master.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> ---
>  elf/Makefile | 22 +++++++++++++---------
>  1 file changed, 13 insertions(+), 9 deletions(-)
> 
> diff --git a/elf/Makefile b/elf/Makefile
> index 632a4d8b0f..60c8082d06 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -482,21 +482,25 @@ $(objpfx)dl-allobjs.os: $(all-rtld-routines:%=$(objpfx)%.os)
>  # are compiled with special flags, and puts these modules into rtld-libc.a
>  # for us.  Then we do the real link using rtld-libc.a instead of libc_pic.a.
>  
> -# If the compiler can do SSP, build the mapfile with dummy __stack_chk_fail
> -# and __stack_chk_fail_local symbols defined, to prevent the real things
> -# being dragged into rtld even though rtld is never built with stack-
> -# protection.
> +# These symbols need to be stubbed out during symbol discovery because
> +# their implementation is provided differently in rtld, and the symbol
> +# discovery mechanism is not compatible with the libc implementation
> +# when compiled for libc.
> +rtld-stubbed-symbols =
> +
> +# The GCC arguments that implement $(rtld-stubbed-symbols).
> +rtld-stubbed-symbols-args = \
> +  $(patsubst %,-Wl$(comma)--defsym=%=0, $(rtld-stubbed-symbols))
>  
>  ifeq ($(have-ssp),yes)
> -dummy-stack-chk-fail := -Wl,--defsym='__stack_chk_fail=0' \
> -			-Wl,--defsym='__stack_chk_fail_local=0'
> -else
> -dummy-stack-chk-fail :=
> +# rtld is not built with the stack protector, so these references will
> +# go away in the rebuilds.
> +rtld-stubbed-symbols += __stack_chk_fail __stack_chk_fail_local
>  endif
>  
>  $(objpfx)librtld.map: $(objpfx)dl-allobjs.os $(common-objpfx)libc_pic.a
>  	@-rm -f $@T
> -	$(reloc-link) -o $@.o $(dummy-stack-chk-fail) \
> +	$(reloc-link) -o $@.o $(rtld-stubbed-symbols-args) \
>  		'-Wl,-(' $^ -lgcc '-Wl,-)' -Wl,-Map,$@T
>  	rm -f $@.o
>  	mv -f $@T $@
>