What you observe results when some of the global symbol definitions in some of
the object files archived in libxxx.a were compiled with the function attribute
or variable attribute visibility("hidden")
This attribute has the effect that when the object file containing the
the global symbol definition is linked into a shared library:
- The linkage of the symbol is changed from global to local in the static symbol table (
.symtab) of the output shared library,
so that when that shared library is linked with anything else, the linker cannot see the definition of the symbol.
- The symbol definition is not added to the dynamic symbol table (
.dynsym) of the output shared library (which by default it would be)
so that when the shared library is loaded into a process, the loader is likewise unable to find a definition of the symbol.
In short, the global symbol definition in the object file is hidden for the purposes of dynamic linkage.
Check this out with:
$ readelf -s libxxx.a | grep HIDDEN
and I expect you to get hits for the unexported global symbols. If you don't,
you need read no further because I have no other explanation of what you see
and wouldn't count on any workaround I suggested not to shoot you in the foot.
Here is an illustration:
a.c
#include <stdio.h>
void aa(void)
{
puts(__func__);
}
b.c
#include <stdio.h>
void __attribute__((visibility("hidden"))) bb(void)
{
puts(__func__);
}
de.c
#include <stdio.h>
void __attribute__((visibility("default"))) dd(void)
{
puts(__func__);
}
void ee(void)
{
puts(__func__);
}
We'll compile a.c and b.c like so:
$ gcc -Wall -c a.c b.c
And we can see that symbols aa and ab are defined and global in their respective object files:
$ nm --defined-only a.o b.o
a.o:
0000000000000000 T aa
0000000000000000 r __func__.2361
b.o:
0000000000000000 T bb
0000000000000000 r __func__.2361
But we can also observe this difference:
$ readelf -s a.o
Symbol table '.symtab' contains 13 entries:
Num: Value Size Type Bind Vis Ndx Name
...
10: 0000000000000000 19 FUNC GLOBAL DEFAULT 1 aa
...
as compared with:
$ readelf -s b.o
Symbol table '.symtab' contains 13 entries:
Num: Value Size Type Bind Vis Ndx Name
...
10: 0000000000000000 19 FUNC GLOBAL HIDDEN 1 bb
...
aa is a GLOBAL symbol with DEFAULT visibility and bb is a GLOBAL
symbol with HIDDEN visibility.
We'll compile de.c differently:
$ gcc -Wall -fvisibility=hidden -c de.c
Here, we're instructing the compiler that any symbol shall be given hidden
visibility unless a countervailing visibility attribute is specified for
it in the source code. And accordingly we see:
$ readelf -s de.o
Symbol table '.symtab' contains 15 entries:
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
...
11: 0000000000000000 19 FUNC GLOBAL DEFAULT 1 dd
...
14: 0000000000000013 19 FUNC GLOBAL HIDDEN 1 ee
Archiving these object files in a static library changes them in no way:
$ ar rcs libabde.a a.o b.o de.o
And then if we link all of them into a shared library:
$ gcc -o libabde.so -shared -Wl,--whole-archive libabde.a -Wl,--no-whole-archive
we find that:
$ readelf -s libabde.so | egrep '(aa|bb|dd|ee|Symbol table)'
Symbol table '.dynsym' contains 8 entries:
6: 0000000000001105 19 FUNC GLOBAL DEFAULT 12 aa
7: 000000000000112b 19 FUNC GLOBAL DEFAULT 12 dd
Symbol table '.symtab' contains 59 entries:
45: 0000000000001118 19 FUNC LOCAL DEFAULT 12 bb
51: 000000000000113e 19 FUNC LOCAL DEFAULT 12 ee
54: 0000000000001105 19 FUNC GLOBAL DEFAULT 12 aa
56: 000000000000112b 19 FUNC GLOBAL DEFAULT 12 dd
bb and ee, which were GLOBAL with HIDDEN visibility in the object files,
are LOCAL in the static symbol of libabde.so and are absent altogether
from its dynamic symbol table.
In this light, you may wish to re-evaluate your mission:
The symbols that have been given hidden visibility in the object files in libxxx.a have
been hidden because the person who compiled them had a reason for
wishing to conceal them from dynamic linkage. Do you have a countervailing need
to export them for dynamic linkage? Or do you maybe just want to export them because
you've noticed that they're not exported and don't know why not?
If you nonetheless want to unhide the hidden symbols, and cannot change the source code
of the object files archived in libxxx.a, your least worst resort is to:
- Extract each object file from
libxxx.a
- Doctor it to replace
HIDDEN with DEFAULT visibility on its global definitions
- Put it into a new archive
libyyy.a
- Then use
libyyy.a instead of libxxx.a.
The binutils tool for doctoring object files is objcopy.
But objcopy has no operations to directly manipulate the dynamic visibility of
a symbol and you'd have to settle for a circuitous kludge that "achieves the effect
of" unhiding the hidden symbols:
- With
objcopy --redefine-sym, rename each hidden global symbol S as, say, __hidden__S.
- With
objcopy --add-symbol, add a new global symbol S that has the same value as __hidden_S
but gets DEFAULT visibility by default.
ending up with two symbols with the same definition: the original hidden one
and a new unhidden alias for it.
Preferable to that would a means of simply and solely changing the visibility of a symbol in
an ELF object file, and a means is to hand in the LIEF library (Library to Instrument Executable Formats) -
Swiss Army Chainsaw for object and executable file alterations1.
Here is a Python script that calls on pylief, the LIEF Python module, to unhide the
hidden globals in an ELF object file:
unhide.py
#!/usr/bin/python
# unhide.py - Replace hidden with default visibility on global symbols defined
# in an ELF object file
import argparse, sys, lief
from lief.ELF import SYMBOL_BINDINGS, SYMBOL_VISIBILITY, SYMBOL_TYPES
def warn(msg):
sys.stderr.write("WARNING: " + msg + "\n")
def unhide(objfile_in, objfile_out = None, namedsyms=None):
if not objfile_out:
objfile_out = objfile_in
binary = lief.parse(objfile_in)
allsyms = { sym.name for sym in binary.symbols }
selectedsyms = set([])
nasyms = { sym.name for sym in binary.symbols if \
sym.type == SYMBOL_TYPES.NOTYPE or \
sym.binding != SYMBOL_BINDINGS.GLOBAL or \
sym.visibility != SYMBOL_VISIBILITY.HIDDEN }
if namedsyms:
namedsyms = set(namedsyms)
nosyms = namedsyms - allsyms
for nosym in nosyms:
warn("No symbol " + nosym + " in " + objfile_in + ": ignored")
for sym in namedsyms & nasyms:
warn("Input symbol " + sym + \
" is not a hidden global symbol defined in " + objfile_in + \
": ignored")
selectedsyms = namedsyms - nosyms
else:
selectedsyms = allsyms
selectedsyms -= nasyms
unhidden = 0;
for sym in binary.symbols:
if sym.name in selectedsyms:
sym.visibility = SYMBOL_VISIBILITY.DEFAULT
unhidden += 1
print("Unhidden: " + sym.name)
print("{} symbols were unhidden".format(unhidden))
binary.write(objfile_out)
def get_args():
parser = argparse.ArgumentParser(
description="Replace hidden with default visibility on " + \
"global symbols defined in an ELF object file.")
parser.add_argument("ELFIN",help="ELF object file to read")
parser.add_argument("-s","--symbol",metavar="SYMBOL",action="append",
help="Unhide SYMBOL. " + \
"If unspecified, unhide all hidden global symbols defined in ELFIN")
parser.add_argument("--symfile",
help="File of whitespace-delimited symbols to unhide")
parser.add_argument("-o","--out",metavar="ELFOUT",
help="ELF object file to write. If unspecified, rewrite ELFIN")
return parser.parse_args()
def main():
args = get_args()
objfile_in = args.ELFIN
objfile_out = args.out
symlist = args.symbol
if not symlist:
symlist = []
symfile = args.symfile
if symfile:
with open(symfile,"r") as fh:
symlist += [word for line in fh for word in line.split()]
unhide(objfile_in,objfile_out,symlist)
main()
Usage:
$ ./unhide.py -h
usage: unhide.py [-h] [-s SYMBOL] [--symfile SYMFILE] [-o ELFOUT] ELFIN
Replace hidden with default visibility on global symbols defined in an ELF
object file.
positional arguments:
ELFIN ELF object file to read
optional arguments:
-h, --help show this help message and exit
-s SYMBOL, --symbol SYMBOL
Unhide SYMBOL. If unspecified, unhide all hidden
global symbols defined in ELFIN
--symfile SYMFILE File of whitespace-delimited symbols to unhide
-o ELFOUT, --out ELFOUT
ELF object file to write. If unspecified, rewrite
ELFIN
And here is a shell script:
unhide.sh
#!/bin/bash
OLD_ARCHIVE=$1
NEW_ARCHIVE=$2
OBJS=$(ar t $OLD_ARCHIVE)
for obj in $OBJS; do
rm -f $obj
ar xv $OLD_ARCHIVE $obj
./unhide.py $obj
done
rm -f $NEW_ARCHIVE
ar rcs $NEW_ARCHIVE $OBJS
echo "$NEW_ARCHIVE made"
that takes:
$1 = Name of an existing static library$2 = Name for a new static library
and creates $2 containing the object files from $1, each modified
with unhide.py to unhide all of its hidden global definitions.
Back with our illustration, we can run:
$ ./unhide.sh libabde.a libnew.a
x - a.o
0 symbols were unhidden
x - b.o
Unhidden: bb
1 symbols were unhidden
x - de.o
Unhidden: ee
1 symbols were unhidden
libnew.a made
and confirm that worked with:
$ readelf -s libnew.a | grep HIDDEN; echo Done
Done
$ readelf -s libnew.a | egrep '(aa|bb|dd|ee)'
10: 0000000000000000 19 FUNC GLOBAL DEFAULT 1 aa
10: 0000000000000000 19 FUNC GLOBAL DEFAULT 1 bb
11: 0000000000000000 19 FUNC GLOBAL DEFAULT 1 dd
14: 0000000000000013 19 FUNC GLOBAL DEFAULT 1 ee
Finally if we relink the shared library with the new archive
$ gcc -o libabde.so -shared -Wl,--whole-archive libnew.a -Wl,--no-whole-archive
all of the global symbols from the archive are exported:
$ readelf --dyn-syms libabde.so | egrep '(aa|bb|dd|ee)'
6: 0000000000001105 19 FUNC GLOBAL DEFAULT 12 aa
7: 000000000000112b 19 FUNC GLOBAL DEFAULT 12 dd
8: 0000000000001118 19 FUNC GLOBAL DEFAULT 12 bb
9: 000000000000113e 19 FUNC GLOBAL DEFAULT 12 ee
[1]
Download C/C++/Python libraries
Debian/Ubuntu provides C/C++ dev package lief-dev.
