Suppress the fetch of an archive member via --defsym (glibc/elf/librtld.map.o)

[Fangrui Song <maskray@google.com>]

cd /tmp/p
git clone git://sourceware.org/git/glibc.git; cd glibc
mkdir Release; ../configure --prefix=/tmp/opt
make -j

When linking elf/librtld.map.o

% gcc -nostdlib -nostartfiles -r -o /tmp/p/glibc/Release/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,-(' /tmp/p/glibc/Release/elf/dl-allobjs.os /tmp/p/glibc/Release/libc_pic.a -lgcc '-Wl,-)' -Wl,-Map,/tmp/p/glibc/Release/elf/librtld.mapT

Without -Wl,defsym:

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

glibc/elf/Makefile uses -Wl,--defsym= (rtld-stubbed-symbols) to suppress libc_pic.a(malloc.os):

% readelf -s elf/librtld.map.o | grep ABS | grep -v LOCAL
    712: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  ABS __stack_chk_fail_local
    826: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  ABS malloc
    876: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  ABS __stack_chk_fail
    905: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  ABS calloc
    975: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  ABS realloc
   1174: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT  ABS free

My question is: does the suppression via --defsym work reliably?

# a.o
call foo

# b.a(b.o)
.globl foo, free
foo:
free:


# GNU ld --defsym is order dependent.
ld.bfd a.o b.a --defsym foo=0  # b.a(b.o) is fetched. free is present
ld.bfd --defsym foo=0 a.o b.a  # b.a(b.o) is not fetched. free is absent

# gold --defsym is order independent. For the more complex glibc elf/librtld.map.o case, it happens to match GNU ld.
gold a.o b.a --defsym foo=0    # b.a(b.o) is not fetched. free is absent
gold --defsym foo=0 a.o b.a    # b.a(b.o) is not fetched. free is absent

# lld --defsym is order independent. --defsym is processed the last. For elf/librtld.map.o it will report a multiple definition error.
# https://sourceware.org/pipermail/libc-alpha/2020-March/111899.html is required to bypass a configure check
ld.lld a.o b.a --defsym foo=0  # b.a(b.o) is not fetched. free is absent
ld.lld --defsym=0 a.o b.a      # b.a(b.o) is not fetched. free is absent