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

[H.J. Lu]

On Sun, Mar 15, 2020 at 10:02 PM Fangrui Song via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On 2020-03-15, Fangrui Song wrote:
> >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
>
> Sorry, clarify the behavior of lld.
>
> # lld --defsym is order independent. --defsym is processed the last. For elf/librtld.map.o it will report a multiple definition error.
> ld.lld a.o b.a --defsym foo=0  # b.a(b.o) is fetched. free is present
> ld.lld --defsym=0 a.o b.a      # b.a(b.o) is fetched. free is present

Glibc build requires a linker compatible with ld.  Can you provide an lld
option to make lld compatible with ld for cases like this?

-- 
H.J.