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

[H.J. Lu]

On Mon, Mar 16, 2020 at 8:47 AM Fangrui Song <maskray@google.com> wrote:
>
> On 2020-03-16, H.J. Lu wrote:
> >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

Gold is irrelevant here since it isn't supported to build glibc:

https://sourceware.org/bugzilla/show_bug.cgi?id=24148

There are also other gold bugs which may impact glibc build.   Please remove
gold from this discussion.

-- 
H.J.