Some time ago I wrote a description of the
-Bsymbolic
linker flag which could do with some further explanation. The
original
article is a good starting place.
One interesting point that I didn't go into was the potential for
code optimisation
-Bsymbolic brings about. I'm not sure if I
missed that at the time, or the toolchain changed, both are probably
equally likely!
Let me recap the example...
ianw@jj:/tmp/bsymbolic$ cat Makefile
all: test test-bsymbolic
clean:
rm -f *.so test testsym
liboverride.so : liboverride.c
$(CC) -Wall -O2 -shared -fPIC -o liboverride.so $<
libtest.so : libtest.c
$(CC) -Wall -O2 -shared -fPIC -o libtest.so $<
libtest-bsymbolic.so : libtest.c
$(CC) -Wall -O2 -shared -fPIC -Wl,-Bsymbolic -o $@ $<
test : test.c libtest.so liboverride.so
$(CC) -Wall -O2 -L. -Wl,-rpath=. -ltest -o $@ $<
test-bsymbolic : test.c libtest-bsymbolic.so liboverride.so
$(CC) -Wall -O2 -L. -Wl,-rpath=. -ltest-bsymbolic -o $@ $<
$ cat liboverride.c
#include <stdio.h>
int foo(void)
printf("override foo called\n");
return 0;
$ cat libtest.c
#include <stdio.h>
int foo(void)
printf("libtest foo called\n");
return 1;
int test_foo(void)
return foo();
$ cat test.c
#include <stdio.h>
int test_foo(void);
int main(void)
printf("%d\n", test_foo());
return 0;
In words;
libtest.so provides
test_foo(), which
calls
foo() to do the actual work.
libtest-bsymbolic.so is simply built with the flag in
question,
-Bsymbolic.
liboverride.so provides the
alternative version of
foo() designed to override the
original via a
LD_PRELOAD of the library.
test is built against
libtest.so,
test-bsymbolic against
libtest-bsymbolic.so.
Running the examples, we can see that the
LD_PRELOAD does
not override the symbol in the library built with
-Bsymbolic.
$ ./test
libtest foo called
1
$ ./test-bsymbolic
libtest foo called
1
$ LD_PRELOAD=liboverride.so ./test
override foo called
0
$ LD_PRELOAD=liboverride.so ./test-bsymbolic
libtest foo called
1
There are a couple of things going on here. Firstly, you can see
that the SYMBOLIC flag is set in the dynamic section, leading to the
dynamic linker behaviour I explained in the
original
article:
ianw@jj:/tmp/bsymbolic$ readelf --dynamic ./libtest-bsymbolic.so
Dynamic section at offset 0x550 contains 22 entries:
Tag Type Name/Value
0x00000001 (NEEDED) Shared library: [libc.so.6]
0x00000010 (SYMBOLIC) 0x0
...
However, there is also an effect on generated code. Have a look at
the PLTs:
$ objdump --disassemble-all ./libtest.so
Disassembly of section .plt:
[... blah ...]
0000039c <foo>:
39c: ff a3 10 00 00 00 jmp *0x10(%ebx)
3a2: 68 08 00 00 00 push $0x8
3a7: e9 d0 ff ff ff jmp 37c <_init+0x30>
$ objdump --disassemble-all ./libtest-bsymbolic.so
Disassembly of section .plt:
00000374 <__gmon_start__@plt-0x10>:
374: ff b3 04 00 00 00 pushl 0x4(%ebx)
37a: ff a3 08 00 00 00 jmp *0x8(%ebx)
380: 00 00 add %al,(%eax)
...
00000384 <__gmon_start__@plt>:
384: ff a3 0c 00 00 00 jmp *0xc(%ebx)
38a: 68 00 00 00 00 push $0x0
38f: e9 e0 ff ff ff jmp 374 <_init+0x30>
00000394 <puts>:
394: ff a3 10 00 00 00 jmp *0x10(%ebx)
39a: 68 08 00 00 00 push $0x8
39f: e9 d0 ff ff ff jmp 374 <_init+0x30>
000003a4 <__cxa_finalize@plt>:
3a4: ff a3 14 00 00 00 jmp *0x14(%ebx)
3aa: 68 10 00 00 00 push $0x10
3af: e9 c0 ff ff ff jmp 374 <_init+0x30>
Notice the difference? There is no PLT entry for
foo()
when
-Bsymbolic is used.
Effectively, the toolchain has noticed that
foo() can
never be overridden and optimised out the PLT call for it. This is
analogous to using "hidden" attributes for symbols, which I have
detailed in another article on
symbol
visiblity attributes (which also goes into PLT's, if the above
meant nothing to you).
So
-Bsymbolic does have some more side-effects than just
setting a flag to tell the dynamic linker about binding rules -- it
can actually result in optimised code. However, I'm still struggling
to find good use-cases for
-Bsymbolic that can't be better
done with Version scripts and visibility attributes. I would
certainly recommend using these methods if at all possible.
Thanks to Ryan Lortie for comments on the original article.
Ubuntu always gets a lot of coverage in blogs, and the recent 
then they are really worth the extra expense, I have no regrets at all about spending about $320US (including tax) on my Bose QC-15. The Bauhn product is good for when you want something cheap, for example a set to be used in a server room, or for the use of children. I bought a Bauhn headset for a friend who is a pilot, he spent $1,100 on a noise-canceling headset for his plane but had never got around to buying one for recreational use I expect that he will allow his children to use his new Bauhn headphones, if they get broken it s only a $69 expense.
The second cheapest NC headphones I ve seen on offer in Australia is 
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Today is March 24th, that means 
i'm aware that people don't always take proper measures during mass OpenPGP keysignings. Apparently, some keys even get signed with no one at the keysigning present speaking for that key (for example, if the key was submitted to the keysigning via online mechanisms beforehand, but the keyholder failed to show up). Unverified certifications are potentially erroneous, and erroneous certifications are bad for the OpenPGP web of trust. Debian and other projects rely on the OpenPGP web of trust being reasonable and healthy. People should make a habit of doing proper verifications at keysignings. People who make unverified certifications should probably be made aware of better practices. So for future keysignings, i may introduce a key to the set under consideration and see what sort of OpenPGP certifications that key receives. I won't pretend to hold that key in person, won't speak for it, and it won't have my name attached to it. But it may be on the list. Depending on the certifications received on that key (and the feedback i get on this blog post), i'll either publish the list of wayward certifiers, or contact the certifiers privately. Wayward certifiers should review their keysigning practices and revoke any certifications they did not adequately verify. Remember, at a keysigning party, for each key: 
The shiny new 0.2.2 release of
by Michael Parenti. It seems that Caesar was more of a populist than a despot.
On Wednesday, I went to a meeting of the Business Initiative for North Somerset for 
I often find myself running /sbin/route to get the IP address of the current gateway, especially when using a wireless LAN while traveling. For example, if the Internet does not work I usually ping the local gateway to see where the connectivity problem lies. I also need the IP if I want to access the routers configuration web interface. This is somewhat tedious, so I wrote 
We had a visitor on #ironruby today asking for help getting IR running on his mac. I gave him the following directions, and they seemed to work aside from one glitch. I tested them on my wife s mac, and it worked for me, too.
Install Mono
You can grab the Mono .dmg from 
We received our US census form the other day. Sarah's already filled it out,
but I wanted to look at it before we mailed it back, just out of curiosity.
I'm astounded at how incredibly basic it is. Literally all it asks is name,
age, date of birth and race. It's somewhat laughable how you're either
"white", or one of a bazillion other racial ethnicities. They don't seem to
be interested that I'm Australian. Or if I were a white Samoan, for example.
I can only remember having filled out one Australian census since I moved
out of home, which was the 2001 census. I missed the 2006 census since I was
living in the US. The US census seems to be a 10 year affair, compared to
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"Census night" was always a big deal in Australia. You were supposed to fill
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The US census form claims to care about the state of affairs on April 1, but
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Every once in a while someone asks how they can use the Debian installer
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I have just sent the following email to my MP, David Lepper MP, outlining my concerns about the Digital Economy Bill. I urge you to write to your MP with a similar letter.
I'm afraid that in the end I failed the challenge. As the last minutes of Saturday night ticked down, I was still mired in the minutiae of C++ Run-Time Type Identification (RTTI.) Sunday was my 12th wedding anniversary and I know from past experience that this is one day when it is advisable to step away from the computer. So I have to concede defeat for now. But I only actually spent about 48 hours on this project this week so if you really think about it, I still have 120 hours available. That's what I choose to believe anyway.
The galling thing is in theory I know how this is supposed to work.