[glibc/maskray/lld] elf: Replace a --defsym trick with an object file to be compatible with lld

[Fangrui Song <maskray@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=a9467e5d34026ad7b242513f5398a89e874fd6e7

commit a9467e5d34026ad7b242513f5398a89e874fd6e7
Author: Fangrui Song <maskray@google.com>
Date:   Sat Apr 11 14:21:35 2020 -0700

    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.

Diff:
---
 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 -
+	$(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.