GNU ld: Uninitialized variables with "-Wl,--exclude-libs,ALL" global

[Johan Bolmsjö]

Hello,

I'm building a shared library using a bunch of static libraries. The shared
library is linked using the option "-Wl,--exclude-libs,ALL". According to
the documentation this option should prevent all symbols in the static
libraries from being exported. I have found this to be true for functions
and initialized variables. Uninitialized variables however, e.g. a global
variable "int foo;" are still global in the resulting shared library. The
gold linker behaves according to the documentation and uninitialized
variables are local in the resulting shared library. I.e. there is a
difference in behaviour between GNU ld and GNU gold linker.
*--exclude-libs* *lib**,**lib**,...* Specifies a list of archive libraries
from which symbols should not be automatically exported.gcc -Wl,--version
collect2 version 4.9.2
/usr/bin/ld -plugin /usr/lib/gcc/x86_64-linux-gnu/4.9/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-linux-gnu/4.9/lto-wrapper
-plugin-opt=-fresolution=/tmp/ccannlod.res -plugin-opt=-pass-through=-lgcc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lgcc_s
--sysroot=/ --build-id --eh-frame-hdr -m elf_x86_64 --hash-style=gnu
-dynamic-linker /lib64/ld-linux-x86-64.so.2
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crt1.o
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/4.9/crtbegin.o
-L/usr/lib/gcc/x86_64-linux-gnu/4.9
-L/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu
-L/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../../lib -L/lib/x86_64-linux-gnu
-L/lib/../lib -L/usr/lib/x86_64-linux-gnu -L/usr/lib/../lib
-L/usr/lib/gcc/x86_64-linux-gnu/4.9/../../.. --version -lgcc --as-needed
-lgcc_s --no-as-needed -lc -lgcc --as-needed -lgcc_s --no-as-needed
/usr/lib/gcc/x86_64-linux-gnu/4.9/crtend.o
/usr/lib/gcc/x86_64-linux-gnu/4.9/../../../x86_64-linux-gnu/crtn.o
GNU ld (GNU Binutils for Debian) 2.25
Copyright (C) 2014 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or (at your option) a later
version.
This program has absolutely no warranty.

I'm using Debian 8 and this is the Debian gcc package. They may have
applied patches for binutils, but probably nothing that should explain this
behaviour.

The following script illustrates the issue.
It's simple enough to be an instruction about what commands to run.
The script does not need to be executed if there are concerns about its
safety.

#!/bin/bash

set -ve

TMP_C="tmp.$$.c"
TMP_O="tmp.$$.o"
TMP_A="libtmp.$$.a"
TMP_GNU_SO="libtmp.$$.gnu.so"
TMP_GOLD_SO="libtmp.$$.gold.so"

cleanup() {
    rm -f $TMP_C $TMP_O $TMP_A $TMP_GNU_SO $TMP_GOLD_SO
}
trap cleanup EXIT

cat > $TMP_C <<EOF
int foo;
int bar = 42;
EOF

# Create static library with two global symbols.
gcc -g3 -Wall -c $TMP_C -o $TMP_O
rm -f $TMP_A
ar rcs $TMP_A $TMP_O

# Create a sharerd library with "-Wl,--exclude-libs,ALL" and the GNU ld
linker.
gcc -shared -Wl,-soname,libtmp.so.1 -o $TMP_GNU_SO -Wl,--exclude-libs,ALL
-L. -Wl,--whole-archive -ltmp.$$ -Wl,--no-whole-archive

# Create a sharerd library with "-Wl,--exclude-libs,ALL" and the GNU gold
linker.
gcc -fuse-ld=gold -shared -Wl,-soname,libtmp.so.1 -o $TMP_GOLD_SO
-Wl,--exclude-libs,ALL -L. -Wl,--whole-archive -ltmp.$$
-Wl,--no-whole-archive

# Show the result.

echo "### GNU ld global symbols"
nm -D $TMP_GNU_SO

echo "### GNU gold global symbols"
nm -D $TMP_GOLD_SO


# Script in uuencoded form
begin 644 -
M(R$O8FEN+V)A<address@hidden"G-E="`M=F4*"E1-4%]#/2)T;7`N)"0N8R(*5$U07T\]
M(G1M<"XD)"YO(@I435!?03TB;&EB=&UP+B0D+F$B"E1-4%]'3E5?4T\](FQI
M8G1M<"XD)"YG;G4N<V\B"E1-4%]'3TQ$7U-//2)L:6)T;7`N)"0N9V]L9"YS
M;R(*"F-L96%N=7`H*2!["B`@("!R;2`M9B`D5$U07T,@)%1-4%]/("1435!?
M02`D5$U07T=.55]33R`D5$U07T=/3$1?4T\*?0IT<F%P(&address@hidden)
M5`H*8V%T(#X@)%1-4%]#(#P\14]&"FEN="!F;V\["FEN="!B87(@/2`T,CL*
M14]&"@HC($-R96%T92!S=&%T:6,@;&EB<F%R>2!W:71H('1W;R!G;&]B86P@
M<WEM8F]L<RX*9V-C("UG,R`M5V%L;"`M8R`D5$U07T,@+6\@)%1-4%]/"G)M
M("UF("1435!?00IA<B!R8W,@)%1-4%]!("1435!?3PH*(R!#<F5A=&address@hidden
M:&%R97)D(&QI8G)A<address@hidden:"`B+5=L+"TM97AC;'5D92UL:6)S+$%,3"(@
M86YD('1H92!'3E4@;&0@;&EN:address@hidden,@+7-H87)E9"`M5VPL+7-O;F%M
M92QL:6)T;7`N<V\N,2`M;R`D5$U07T=.55]33R`M5VPL+2UE>&-L=61E+6QI
M8G,L04Q,("U,+B`M5VPL+2UW:&]L92UA<F-H:79E("UL=&UP+B0D("U7;"PM
M+6YO+7=H;VQE+6%R8VAI=F4*"B,@0W)E871E(&$@<VAA<F5R9"!L:6)R87)Y
M('=I=&@@(BU7;"PM+65X8VQU9&4M;&EB<RQ!3$PB(&%N9"!T:&address@hidden(&=O
M;&0@;&EN:address@hidden,@+69U<V4M;&0]9V]L9"`M<VAA<F5D("U7;"PM<V]N
M86UE+&QI8G1M<"YS;RXQ("UO("1435!?1T],1%]33R`M5VPL+2UE>&-L=61E
M+6QI8G,L04Q,("U,+B`M5VPL+2UW:&]L92UA<F-H:79E("UL=&UP+B0D("U7
M;"PM+6YO+7=H;VQE+6%R8VAI=F4*"B,@4VAO=R!T:&4@<address@hidden
M;R`B(R,C($=.52!L9"!G;&]B86P@<WEM8F]L<R(*;address@hidden@)%1-4%]'3E5?
M4T\*"F5C:&\@(B,C(R!'address@hidden"!G;&]B86P@<WEM8F]L<R(*;address@hidden@
-)%1-4%]'3TQ$7U-/"@``
`
end



Script output:

### GNU ld global symbols
000000000020088c B __bss_start
                 w __cxa_finalize
000000000020088c D _edata
0000000000200898 B _end
0000000000000630 T _fini
*0000000000200890 B foo*
                 w __gmon_start__
00000000000004d8 T _init
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable
                 w _Jv_RegisterClasses

### GNU gold global symbols
0000000000001968 A __bss_start
                 w __cxa_finalize
0000000000001968 A _edata
0000000000001970 A _end
0000000000000680 T _fini
                 w __gmon_start__
0000000000000528 T _init
                 w _ITM_deregisterTMCloneTable
                 w _ITM_registerTMCloneTable
                 w _Jv_RegisterClasses