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

["Fāng-ruì Sòng" <maskray@google.com>]

On Mon, Jan 18, 2021 at 4:04 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> 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.

If symbol: and symbol = 0; work, isn't symbol: slightly better because
it looks more normal?
Or is the intention here using SHN_ABS to make it clear these symbols
are special?

> > +       $(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.