Bug 15167 - ld merges gnu_unique def and normal ref into normal symbol
Summary: ld merges gnu_unique def and normal ref into normal symbol
Alias: None
Product: binutils
Classification: Unclassified
Component: ld (show other bugs)
Version: 2.24
: P2 normal
Target Milestone: ---
Assignee: Not yet assigned to anyone
Depends on:
Reported: 2013-02-21 14:30 UTC by Michael Matz
Modified: 2013-02-22 16:25 UTC (History)
2 users (show)

See Also:
Last reconfirmed:


Note You need to log in before you can comment on or make changes to this bug.
Description Michael Matz 2013-02-21 14:30:19 UTC
This came up over at http://gcc.gnu.org/ml/gcc/2013-02/msg00227.html
describing a strange mismatch between two symbols that are unique, but
in the shared object one is normal and the other is unique.  I traced
this back to an ordering problem in ld at http://gcc.gnu.org/ml/gcc/2013-02/msg00239.html .  The binutils involved
there are 2.21.1, but current upstream still exhibits the issue.  I made a
minimal testcase with two asm files:

% cat unique1.s
        .file   "unique.cc"
        .weak   _ZN1SIiE1iE
        .section        .bss._ZN1SIiE1iE,"awG",@nobits,_ZN1SIiE1iE,comdat
        .align 4
        .type   _ZN1SIiE1iE, @gnu_unique_object
        .size   _ZN1SIiE1iE, 4
        .zero   4
        .ident  "GCC: (GNU) 4.6.1 20110505 (prerelease)"
        .section        .note.GNU-stack,"",@progbits

% cat unique2.s
        .file   "unique.cc"
        .align 2
        .globl  _ZN1SIiE3getEv
        .type   _ZN1SIiE3getEv, @function
        pushq   %rbp
        .cfi_def_cfa_offset 16
        .cfi_offset 6, -16
        movq    %rsp, %rbp
        .cfi_def_cfa_register 6
        movq    %rdi, -8(%rbp)
        movq    _ZN1SIiE1iE@GOTPCREL(%rip), %rax
        movl    (%rax), %eax
        popq    %rbp
        .cfi_def_cfa 7, 8
        .size   _ZN1SIiE3getEv, .-_ZN1SIiE3getEv
        .ident  "GCC: (GNU) 4.6.1 20110505 (prerelease)"
        .section        .note.GNU-stack,"",@progbits

% gcc -c unique1.s unique2.s
% ./ld/ld-new -shared -o unique-ok.so unique2.o unique1.o
% ./ld/ld-new -shared -o unique-bad.so unique1.o unique2.o
% nm -D unique-bad.so | grep _ZN1SIiE1iE
0000000000200398 B _ZN1SIiE1iE
% nm -D unique-ok.so | grep _ZN1SIiE1iE
0000000000200398 u _ZN1SIiE1iE

In difference is that for unique-ok.so the reference to _ZN1SIiE1iE comes
first (in unique2.o), then the definition (in unique1.o), and that results
correctly in a global unique symbol exported from the DSO.
For unique-bad.so the definition comes first, and the uniqueness seems to
get lost when the normal reference is seen, so a normal global (not even weak)
symbol is now exported from the DSO.

This can (and as the thread at gcc@ shows actually does) lead to problems
when the system relies on some symbols to be indeed globally unique.
The empty_rep static member of libstdc++'s string<> and wstring<> templates
is one example.

As said, this is broken in at least 2.21.1 and in current trunk.  FWIW,
the C++ source I used to initially generate the above asms is:

template <typename T>
struct S {
    static T i;
    T get();
template <typename T> T S<T>::i;
template <> int S<int>::get() { return i; }

compiled with -fPIC, and then split the .s into two by hand.
Comment 1 H.J. Lu 2013-02-21 19:12:37 UTC
[hjl@gnu-6 pr15167]$ cat unique_shared1.s
	.type b, %gnu_unique_object
b:	.long 0
	.size b, .-b
[hjl@gnu-6 pr15167]$ cat unique_shared2.s
	.dc.a	b
[hjl@gnu-6 pr15167]$ make
as   -o unique_shared1.o unique_shared1.s
as   -o unique_shared2.o unique_shared2.s
./ld -shared -o unique_shared.so unique_shared1.o unique_shared2.o
readelf -s unique_shared.so | grep UNIQ
make: *** [all] Error 1
[hjl@gnu-6 pr15167]$
Comment 2 cvs-commit@gcc.gnu.org 2013-02-22 01:20:55 UTC
CVSROOT:	/cvs/src
Module name:	src
Changes by:	hjl@sourceware.org	2013-02-22 01:20:48

Modified files:
	ld/testsuite   : ChangeLog 
	ld/testsuite/ld-unique: unique.exp 
	bfd            : ChangeLog elf64-ia64-vms.c elflink.c 

Log message:
	Set unique_global only for definition
	PR ld/15167
	* elf64-ia64-vms.c (elf64_vms_link_add_object_symbols): Set
	unique_global only for definition.
	* elflink.c (_bfd_elf_merge_symbol): Don't set unique_global
	(elf_link_add_object_symbols): Set unique_global only
	for definition.
	PR ld/15167
	* ld-unique/unique.exp: Add a test for shared library with

Comment 3 H.J. Lu 2013-02-22 16:25:58 UTC