Shared objects (*.so) in Unix-like systems are inefficient because of the symbol interposition: Every access to a global variable inside the .so needs a GOT lookup, and every call from one function to another inside the .so needs a PLT lookup. I was therefore happy to see that gcc version 5.1 has added the option -fno-semantic-interposition. However, when I try to make a .so where one function calls another without using a PLT, I get the error message:
relocation R_X86_64_PC32 against symbol `functionname' can not be used when making a shared object; recompile with -fPIC
I expected the option -fno-semantic-interposition to eliminate this error message, but it doesn't. -mcmodel=large doesn't help either.
The reference to the function is indeed position-independent, which the error message actually confirms (R_X86_64_PC32 means PC-relative 32-bit relocation in 64-bit mode). -fPIC does not really mean position-independent, as the name would imply, it actually means use GOT and PLT.
I cannot use __attribute__((visibility ("hidden"))) because the called function and the caller are compiled in seperate files (caller is in C++, called function is in assembly).
I tried to make an assembly listing to see what the option -fno-semantic-interposition does. I found out that it makes a reference to a local alias when one function calls another in the same file, but it still uses a PLT when calling a function in another file.
(g++ version is 5.2.1 Ubuntu, 64-bit mode).
Is there a way to make the linker accept a cross-reference inside a .so without the GOT/PLT lookup?
Is there a way to make the linker accept a cross-reference inside a .so without the GOT/PLT lookup?
Yes: attribute((visibility("hidden"))) is exactly the way to do it.
I cannot use attribute((visibility ("hidden"))) because the called function and the caller are compiled in seperate files
You are confused: visibility("hidden") means that the symbol will not be exported from the shared library, when it is finally linked. But the symbol is global and visible across multiple translation units before that final link.
Proof:
$ cat t1.c
extern int foo() __attribute__((visibility("hidden")));
int main() { return foo(); }
$ cat t2.c
int foo() __attribute__((visibility("hidden")));
int foo() { return 42; }
$ gcc -c -fPIC t1.c t2.c
$ gcc -shared t1.o t2.o -o t.so
$ nm -D t.so | grep foo
$
I tried to make an assembly listing to see what the option -fno-semantic-interposition does. I found out that it makes a reference to a local alias when one function calls another in the same file, but it still uses a PLT when calling a function in another file.
If you read the discussion in gcc-patches, you'll see that the -fno-semantic-interposition is about allowing inlining of possibly interposable functions, not about the way they are actually called when not inlined.
Related
I have downloaded libgcrypt library source code and I want to customize this standard shared library by adding my function inside one particular
source code file .
Although compilation/build process of customized shared library is successful, but it shows error at linking time.
Here is what I have done .
inside /src/visibility.c file, I have added my custom function,
void MyFunction(void)
{
printf("This is added just for testing purpose");
}
I have also include function prototype inside /src/gcrypt.h
void MyFunction(void);
#build process
./configure --prefix=/usr
sudo make install
nm command find this custom function.
nm /usr/lib/libgcrypt.so | grep MyFunction
000000000000dd70 t MyFunction
Here is my sample code to access my custom function.
//aes_gcrypt_example.c
#include <stdio.h>
#include <gcrypt.h>
#include <assert.h>
int main()
{
MyFunction();
return 0;
}
gcc aes_gcrypt_example.c -o aes -lgcrypt
/tmp/ccA0qgAB.o: In function `main':
aes_gcrypt_example.c:(.text+0x3a2): undefined reference to `MyFunction'
collect2: error: ld returned 1 exit status
I also tried by making MyFunction as extern inside gcrypt.h, but in that case also I am getting same error.
Why is this happening ?
Is the customization of standard library is not allowed ?
If YES, then is there any FLAG to disable to allow customization ?
If NO, what mistake I am making ?
It would be great help if someone provide some useful link/solution for the above mentioned problem. I am using Ubuntu16.04 , gcc 4.9.
Lower-case t for the symbol type?
nm /usr/lib/libgcrypt.so | grep MyFunction
000000000000dd70 t MyFunction
Are you sure that's a visible function? On my Ubuntu 16.04 VM, the linkable functions defined in an object file have T (not t) as the symbol type. Is there a stray static kicking around and causing confusion? Check a couple of other functions defined in libgcrypt.so (and documented in gcrypt.h) and see whether they have a t or a T. They will have a T and not t. You'll need to work out why your function gets a t — it is not clear from the code you show.
The (Ubuntu) man page for nm includes:
The symbol type. At least the following types are used; others
are, as well, depending on the object file format. If lowercase,
the symbol is usually local; if uppercase, the symbol is global
(external).
The line you show says that MyFunction is not visible outside its source file, and the linker agrees because it is not finding it.
Your problem now is to check that the object file containing MyFunction has a symbol type T — if it doesn't the problem is in the source code.
Assuming that the object file shows symbol type T but the shared object shows symbol type t, you have to find what happens during the shared object creation phase to make the symbol invisible outside the shared object. This is probably because of a 'linker script' that controls which symbols are visible outside the library (or maybe just compilation options). You can search on Google with 'linker script' and various extra words ('tutorial', 'provide', 'example', etc) and come up with links to the relevant documentation.
You may need to research documentation for LibTool, or for the linker BinUtils. LibTool provides ways of manipulating shared libraries. In a compilation command line that you show in a comment, there is the option -fvisibility=hidden. I found (mostly by serendipitous accident) a GCC Wiki on visibility. See also visibility attribute and code generation options.
Say I have an address acquired by __builtin_return_address(0).
Sometime later I want to translate it to a coreesponding function name.
There's dladdr(3) which works only for dynamic libraries.
Is there any way to receive function of any symbol (maybe by libunwind, backrace_symbol() etc.)?
Compile all your code (and perhaps even some shared libraries that you use) with debug info (-g). Notice that GCC enables to use both -g and some optimization flag like -O2 (of course, in that case, the debug info is "approximate"). So you can compile code with gcc -Wall -g -O2 etc....
Then use perhaps Ian Taylor's libbacktrace which is included in recent versions of GCC.
BTW, dladdr or backtrace_symbol might also work (but I recommend using libbacktrace because it is parsing DWARF info). And dladdr(3) does work on symbols from the executable itself. You may need to link your executable with -rdynamic flag.
Notice also that static symbols (notably static functions) "don't really exist" in the ELF executable (only global symbols are kept in it) so dladdr cannot give them. Be also aware of the visibility function attribute and pragma.
I have a code A that is statically linked against one version of mpich. Now comes library B, which is used by A via dlopen(). B depends on mpich as well, but is linked dynamically against it.
The problem is that now, in order for B to take advantage of mpi distribution, needs to access the communicator currently handled by A. This communicator has been created by A static version of mpich, When B invokes MPI routines, it will use a dynamic version of MPI which is not guarateed to be compatible with the static version attached to A.
This is the overall picture. I think that the only solution is to have mpich dynamically linked for both A and B. What I am not fully understanding is however the following:
how does the linker handle shared objects dependencies when dlopening? Will I have two instances of mpich in VM also with dynamic linking, or is the linker smart enough to realize that the symbols required by the dlopened B are already in the address space and will resolve against those.
Is it possible to tell the linker: when you dlopen this library, don't go fetch the dynamic dependency, but resolve it with the static symbols that are already provided by A
In short: it depends on dlopen options. By default, if a symbol needed by the requested library already exists in the global scope, it will be reused (this is what you want). But you can bypass this behavior with RTLD_DEEPBIND, with this flag, the dependencies won't be reused from the global scope, and will be loaded a second time.
Here is some code to reproduce your situation and demo the effect of this flag.
Let's make a common library that will be used by both lib A and program B. This library will exist in two versions.
$ cat libcommon_v1.c
int common_func(int a)
{
return a+1;
}
$ cat libcommon_v2.c
int common_func(int a)
{
return a+2;
}
Now let's write lib A that uses libcommon_v2:
$ cat liba.c
int common_func(int a);
int a_func(int a)
{
return common_func(a)+1;
}
And finally program B that dynamically links to libcommon_v1 and dlopens lib A:
$ cat progb.c
#include <stdio.h>
#include <dlfcn.h>
int common_func(int a);
int a_func(int a);
int main(int argc, char *argv[])
{
void *dl_handle;
int (*a_ptr)(int);
char c;
/* just make sure common_func is registered in our global scope */
common_func(42);
printf("press 1 for global scope lookup, 2 for deep bind\n");
c = getchar();
if(c == '1')
{
dl_handle = dlopen("./liba.so", RTLD_NOW);
}
else if(c == '2')
{
dl_handle = dlopen("./liba.so", RTLD_NOW | RTLD_DEEPBIND);
}
else
{
printf("wrong choice\n");
return 1;
}
if( ! dl_handle)
{
printf("dlopen failed: %s\n", dlerror());
return 2;
}
a_ptr = dlsym(dl_handle, "a_func");
if( ! a_ptr)
{
printf("dlsym failed: %s\n", dlerror());
return 3;
}
printf("calling a_func(42): %d\n", (*a_ptr)(42));
return 0;
}
Let's build and run all the things:
$ export LD_LIBRARY_PATH=.
$ gcc -o libcommon_v1.so -fPIC -shared libcommon_v1.c
$ gcc -o libcommon_v2.so -fPIC -shared libcommon_v2.c
$ gcc -Wall -g -o progb progb.c -L. -lcommon_v1 -ldl
$ gcc -o liba.so -fPIC -shared liba.c -L. -lcommon_v2
$ ./progb
press 1 for global scope lookup, 2 for deep bind
1
calling a_func(42): 44
$ ./progb
press 1 for global scope lookup, 2 for deep bind
2
calling a_func(42): 45
We can clearly see that with default options, dlopen reuses the symbol common_func that was present in program B and that with RTLD_DEEPBIND, libcommon was loaded again and library A got its own version of common_func.
You did not say which Toolchain (GCC, LLVM, MSC, etc.) you are using, the most helpful answer will depend upon this information.
May I suggest you look at "GCC Exception Frames" http://www.airs.com/blog/archives/166 .
If that is helpful then the Gold Linker, which is available for GCC and LLVM, supports 'Link Time Optimization' and can run in "Make" with DLLTool http://sourceware.org/binutils/docs/binutils/dlltool.html .
Indeed it is possible to have both Static and Dynamic Code call each other, the Computer does not care; it will 'run' anything it is fed -- whether that ends up working exactly the way you want or HCFs depends on correct Code and correct Linker commands.
Using a Debugger will not be fun. It would be best to mangle the names prior to Linking so that when debugging you can see from which module the Code came. Once it is up and running you can undef the Mangle and have the same-named Functions Link (to ensure it still functions).
Compiler / Linker Bugs will not be your friend.
This sort of scenario (Static and Dynamic Linking) occurs more often with MinGW and Cygwin where some of the Libraries are Static yet a Library you download from the Internet is only available in Dynamic form (without Source).
If the Library is from two different Compiler Toolchains then other issues arise, see this StackOverflow Thread: " linking dilemma (undefined reference) between MinGW and MSVC. MinGW fails MSVC works ".
It would be best to simply get the newest version of the Library from the source and compile the whole thing yourself, rather than rely on trying to cobble together bits and pieces from different sources (though it is possible to do that).
You can even load the Dynamic Library and call it (statically) and then reload portions of it later.
How tight are you on Memory and how fast do you want Functions to run, if everything is in Memory your Program can transfer execution to called Functions straight away, if you are swapping a portion of your Code to VM your execution times will really take a hit.
Running a Profiler on your Code will help decide what portions of a Library to load if you want to do 'dynamic dynamic linking' (full control of your dyna-linking by loading a Dynamic Library so it can be used Statically). This is the stuff that headaches and nightmare are made of. GL.
Short:
I'm looking for something that will list all unresolved dependencies in an SO, taking into account the SOs that are in it's dependencies.
Long:
I'm converting a lot of static-compiled code to Shared Objects in Linux- is there an easy way to determine what other SOs my recently compiled SO is dependent on besides trial & error while trying to load it?
I'm sure there is a better way, but I haven't been able to find it yet.
I've found "ldd", but that only lists what the SO says it's dependent on.
I've also used "nm" to figure out once an SO fails to load to verify what other SO contains it.
I don't have code for you, but I can give pointers:
It's just a graph problem. You should use objdump -T to dump the dynamic symbol table for a given binary or shared object. You'll see many lines of output, and the flags can be a little confusing, but the important part if that symbols will either be *UND* or they'll have a segment name (.text etc).
Any time you see *UND*, that means that it's an undefined symbol which has to be resolved. Defined symbols are the targets of resolution.
With that, and a little Python, you should be able to find what you need.
"ldd -r foo.so" should print the set of symbols which foo.so needs but which aren't provided by its direct dependencies.
Alternatively, link foo.so like this:
gcc -shared -o foo.so foo.o bar.o -ldep1 -ldep2 -Wl,--no-undefined
This should fail (to link) if foo.o or bar.o uses something not provided by libdep1 or libdep2 or libc.
I'm writing some code that uses dynamic shared libraries as plugins.
My command line for building the shared libraries looks like:
cc -shared -fPIC -o module.so -g -Wall module.c
Within the module, I can call functions that are in any other shared library that has been loaded within the main executable.
However I cannot access (exported) functions that are in the executable itself (I get undefined symbol errors).
My call to dlopen looks like this:
void *handle = dlopen(plugin, RTLD_NOW);
Can anyone please advise how my module can call back to my executable, without having to put all of the executable's utility functions into yet another shared library?
Correct solution is to add -rdynamic to the link command of the main executable. This will add appropriate option to ld (which, when using GNU ld, happens to be --export-dynamic).
Adding --export-dynamic directly is technically incorrect: it's a linker option, and so should be added as -Wl,--export-dynamic, or -Wl,-E. This is also less portable than -rdynamic (other linkers have an equivalent, but the option itself is different).
I've found the answer myself.
I had to add the --export-dynamic flags to the link options for the main executable.
When creating a dynamically linked
executable, add all symbols to the
dynamic symbol table. The dynamic
symbol table is the set of symbols
which are visible from dynamic objects
at run time.
If you do not use this option, the
dynamic symbol table will normally
contain only those symbols which are
referenced by some dynamic object
mentioned in the link.
If you use "dlopen" to load a dynamic
object which needs to refer back to
the symbols defined by the program,
rather than some other dynamic object,
then you will probably need to use
this option when linking the program
itself.
When I encountered the same problem, I just used the following solution. Before loading any plugin, just load the program itself, bringing its symbols to dynamic tables:
dlopen(NULL,RTLD_NOW|RTLD_GLOBAL);
I think the solution is better. The reason is that, it also solves the same problem if you
a) your program (or a trird-party module) is linked (not in runtime) against the shared library, which symbols need to be in dynamic table;
b) can not recompile that module with -rdynamic flag.