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 Tue, Jan 19, 2021 at 4:31 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Tue, Jan 19, 2021 at 12:50 AM Fāng-ruì Sòng <maskray@google.com> wrote:
> >
> > 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?
>
> Generate machine independent assembly codes is tricky.   We should
> make before and after the change as close as possible to avoid ANY
> surprises.   BTW, you need to check HAVE_ASM_SET_DIRECTIVE
> to decide using ".set" vs "=".
>
> --
> H.J.

Thanks for the tip. In this case I think the patch as is (sticking
with `foo:`, don't use `foo = 0` or set) is better.
foo: works everywhere.