I'm encountering an issue which has been elaborated in a good article Shared Library Symbol Conflicts (on Linux). The problem is that when the execution and .so have defined the same name functions, if the .so calls this function name, it would call into that one in execution rather than this one in .so itself.
Let's talk about the case in this article. I understand the DoLayer() function in layer.o has an external function dependency of DoThing() when compiling layer.o.
But when compiling the libconflict.so, shouldn't the external function dependency be resolved in-place and just replaced with the address of conflict.o/DoThing() statically?
Why does the layer.o/DoLayer() still use dynamic linking to find DoThing()? Is this a designed behavior?
Is this a designed behavior?
Yes.
At the time of introduction of shared libraries on UNIX, the goal was to pretend that they work just as if the code was in a regular (archive) library.
Suppose you have foo() defined in both libfoo and libbar, and bar() in libbar calls foo().
The design goal was that cc main.c -lfoo -lbar works the same regardless of whether libfoo and libbar are archive or a shared libraries. The only way to achieve this is to have libbar.so use dynamic linking to resolve call from bar() to foo(), despite having a local version of foo().
This design makes it impossible to create a self-contained libbar.so -- its behavior (which functions it ends up calling) depends on what other functions are linked into the process. This is also the opposite of how Windows DLLs work.
Creating self-contained DSOs was not a consideration at the time, since UNIX was effectively open-source.
You can change the rules with special linker flags, such as -Bsymbolic. But the rules get complicated very quickly, and (since that isn't the default) you may encounter bugs in the linker or the runtime loader.
Yes, this is a designed behavior. When you link a program into a binary, all the references to named external (non-static) functions are resolved to point into the symbol table for the binary. Any shared libraries that are linked against are specified as DT_NEEDED entries.
Then, when you run the binary, the dynamic linker loads each required shared library to a suitable address and resolves each symbol to an address. Sometimes this is done lazily, and sometimes it is done once at first startup. If there are multiple symbols with the same name, one of them will be chosen by the linker, and your program will likely crash since you may not end up with the right one.
Note that this is the behavior on Linux, which has all symbols as a flat namespace. Windows resolves symbols differently, using a tree topology, which has both advantages (fewer conflicts) and disadvantages (the inability to allocate memory in one library and free it in another).
The Linux behavior is very important if you want things like LD_PRELOAD to work. This allows you to use debugging tools like Electric Fence and CPU profiling tools like the Google performance tools, or replace a memory allocator at runtime. None of these things would work if symbols were preferentially resolved to their binary or shared library.
The GNU dynamic linker does support symbol versions, however, so that it's possible to load multiple versions of a shared library into the same program. Oftentimes distros like Debian will do this with libraries they expect to change frequently, like OpenSSL. If the program uses liba which uses OpenSSL 1.0 and libb which uses OpenSSL 1.1, then the program should still function in such a case since OpenSSL has versioned symbols, and each library will use the appropriate version of the relevant symbol.
Related
There's a well-known technique for interposing dynamically linked binaries: creating a shared library and and using LD_PRELOAD variable. But it doesn't work for statically-linked binaries.
One way is to write a static library that interpose the functions and link it with the application at compile time. But this isn't practical because re-compiling isn't always possible (think of third-party binaries, libraries, etc).
So I am wondering if there's a way to interpose statically linked binaries in the same LD_PRELOAD works for dynamically linked binaries i.e., with no code changes or re-compilation of existing binaries.
I am only interested in ELF on Linux. So it's not an issue if a potential solution is not "portable".
One way is to write a static library that interpose the functions and link it with the application at compile time.
One difficulty with such an interposer is that it can't easily call the original function (since it has the same name).
The linker --wrap=<symbol> option can help here.
But this isn't practical because re-compiling
Re-compiling is not necessary here, only re-linking.
isn't always possible (think of third-party binaries, libraries, etc).
Third-party libraries work fine (relinking), but binaries are trickier.
It is still possible to do using displaced execution technique, but the implementation is quite tricky to get right.
I'll assume you want to interpose symbols in main executable which came from a static library which is equivalent to interposing a symbol defined in executable. The question thus reduces to whether it's possible to intercept a function defined in executable.
This is not possible (EDIT: at least not without a lot of work - see comments to this answer) for two reasons:
by default symbols defined in executable are not exported so not accessible to dynamic linker (you can alter this via -export-dynamic or export lists but this has unpleasant performance or maintenance side effects)
even if you export necessary symbols, ELF requires executable's dynamic symtab to be always searched first during symbol resolution (see section 1.5.4 "Lookup Scope" in dsohowto); symtab of LD_PRELOAD-ed library will always follow that of executable and thus won't have a chance to intercept the symbols
What you are looking for is called binary instrumentation (e.g., using Dyninst or ptrace). The idea is you write a mutator program that attaches to (or statically rewrites) your original program (called mutatee) and inserts code of your choice at specific points in the mutatee. The main challenge usually revolves around finding those insertion points using the API provided by the instrumentation engine. In your case, since you are mainly looking for static symbols, this can be quite challenging and would likely require heuristics if the mutatee is stripped of non-dynamic symbols.
As stated, I want to be able to check that a shared library, created by libtool, is not missing any symbols,
I have written a library that is built as a shared library, 'A'. It depends in turn on another library 'B'.
The other library 'B' does not follow strict semver, and so sometimes introduces new functions in minor or patch releases.
Although I try to put appropriate #if B_LIB_VERSION >= 42 in the code for my library to not attempt to call a function in library B if it is not going to be available, apparently I sometimes get the version incorrect. This causes an error when the program is run.
Is it possible with libtool, or any other tool, to ask it to produce a list of all the symbols that are not found in a shared library, or any of the libraries that it will load?
As stated, I want to be able to check that a shared library, created by libtool, is not missing any symbols,
That's hard to do with shared libraries, as they are designed to allow for late symbol resolution. If you're not using dlopen type features, you might be able to build static executables from static versions of A and B and look for missing symbols.
The other library 'B' does not follow strict semver, and so sometimes introduces new functions in minor or patch releases.
I'd seriously consider searching for a replacement library, than having to keep on dealing with their dependency issues.
Is it possible with libtool, or any other tool, to ask it to produce a list of all the symbols that are not found in a shared library, or any of the libraries that it will load?
No, not really. nm will give you a list of symbols that are undefined (and referenced) in a shared library. objdump might be of some use also. On linux, ldd might do some of what you want. But generally there is no way of knowing exactly what a shared library loads, even without considering dlopen.
libltdl might be of some use also if you have to stick with the misbehaving library. At least you can figure out at runtime if libB.42 has symbol xyz or not. It's not as easy as the conditional code way of doing things.
How to create a shared object that is statically linked with pthreads and libstdc++ on Linux/gcc?
Before I go to answering your question as it was described, I will note that it is not exactly clear what you are trying to achieve in the end, and there is probably a better solution to your problem.
That said - there are two main problems with trying to do what you described:
One is, that you will need to decompose libpthread and libstdc++ to the object files they are made with. This is because ELF binaries (used on Linux) have two levels of "run time" library loading - even when an executable is statically linked, the loader has to load the statically linked libraries within the binary on execution, and map the right memory addresses. This is done before the shared linkage of libraries that are dynamically loaded (shared objects) and mapped to shared memory. Thus, a shared object cannot be statically linked with such libraries, as at the time the object is loaded, all static linked libraries were loaded already. This is one difference between linking with a static library and a plain object file - a static library is not merely glued like any object file into the executable, but still contains separate tables which are referred to on loading. (I believe that this is in contrast to the much simpler static libraries in MS-DOS and classic Windows, .LIB files, but there may be more to those than I remember).
Of course you do not actually have to decompose libpthread and libstdc++, you can just use the object files generated when building them. Collecting them may be a bit difficult though (look for the objects referred to by the final Makefile rule of those libraries). And you would have to use ld directly and not gcc/g++ to link, to avoid linking with the dynamic versions as well.
The second problem is consequential. If you do the above, you will sure have such a shared object / dynamic library as you asked to build. However, it will not be very useful, as once you try to link a regular executable that uses those libpthread/libstdc++ (the latter being any C++ program) with this shared object, it will fail with symbol conflicts - the symbols of the static libpthread/libstdc++ objects you linked your shared object against will clash with the symbols from the standard libpthread/libstdc++ used by that executable, no matter if it is dynamically or statically linked with the standard libraries.
You could of course then try to either hide all symbols in the static objects from libstdc++/libpthread used by your shared library, make them private in some way, or rename them automatically on linkage so that there will be no conflict. However, even if you get that to work, you will find some undesireable results in runtime, since both libstdc++/libpthread keep quite a bit of state in global variables and structures, which you would now have duplicate and each unaware of the other. This will lead to inconsistencies between these global data and the underlying operating system state such as file descriptors and memory bounds (and perhaps some values from the standard C library such as errno for libstdc++, and signal handlers and timers for libpthread.
To avoid over-broad interpretation, I will add a remark: at times there can be sensible grounds for wanting to statically link against even such basic libraries as libstdc++ and even libc, and even though it is becoming a bit more difficult with recent systems and versions of those libraries (due to a bit of coupling with the loader and special linker tricks used), it is definitely possible - I did it a few times, and know of other cases in which it is still done. However, in that case you need to link a whole executable statically. Static linkage with standard libraries combined with dynamic linkage with other objects is not normally feasible.
Edit: One issue which I forgot to mention but is important to take into account is C++ specific. C++ was unfortunately not designed to work well with the classic model of object linkage and loading (used on Unix and other systems). This makes shared libraries in C++ not really portable as they should be, because a lot of things such as type information and templates are not cleanly separated between objects (often being taken, together with a lot of actual library code at compile time from the headers). libstdc++ for that reason is tightly coupled with GCC, and code compiled with one version of g++ will in general only work with the libstdc++ from with this (or a very similar) version of g++. As you will surely notice if you ever try to build a program with GCC 4 with any non-trivial library on your system that was built with GCC 3, this is not just libstdc++. If your reason for wanting to do that is trying to ensure that your shared object is always linked with the specific versions of libstdc++ and libpthread that it was built against, this would not help because a program that uses a different/incompatible libstdc++ would also be built with an incompatible C++ compiler or version of g++, and would thus fail to link with your shared object anyway, aside from the actual libstdc++ conflicts.
If you wonder "why wasn't this done simpler?", a general rumination worth pondering: For C++ to work nicely with dynamic/shared libraries (meaning compatibility across compilers, and the ability to replace a dynamic library with another version with a compatible interface without rebuilding everything that uses it), not just compiler standartization is needed, but at the level of the operating system's loader, the structure and interface of object and library files and the work of the linker would need to be significantly extended beyond the relatively simple Unix classics used on common operating systems (Microsoft Windows, Mach based systems and NeXTStep relatives such as Mac OS, VMS relatives and some mainframe systems also included) for natively built code today. The linker and dynamic loader would need to be aware of such things as templates and typing, having to some extent functionality of a small compiler to actually adapt the library's code to the type given to it - and (personal subjective observation here) it seems that higher-level intermediate intermediate code (together with higher-level languages and just-in-time compilation) is catching ground faster and likely to be standardized sooner than such extensions to the native object formats and linkers.
You mentioned in a separate comment that you are trying to port a C++ library to an embedded device. (I am adding a new answer here instead of editing my original answer here because I think other StackOverflow users interested in this original question may still be interested in that answer in its context)
Obviously, depending on how stripped down your embedded system is (I have not much embedded Linux experience, so I am not sure what is most likely), you may of course be able to just install the shared libstdc++ on it and dynamically link everything as you would do otherwise.
If dynamically linking with libstdc++ would not be good for you or not work on your system (there are so many different levels of embedded systems that one cannot know), and you need to link against a static libstdc++, then as I said, your only real option is static linking the executable using the library with it and libstdc++. You mentioned porting a library to the embedded device, but if this is for the purpose of using it in some code you write or build on the device and you do not mind a static libstdc++, then linking everything statically (aside from perhaps libc) is probably OK.
If the size of libstdc++ is a problem, and you find that your library is actually only using a small part of its interfaces, then I would nonetheless suggest first trying to determine the actual space you would save by linking against only the parts you need. It may be significant or not, I never looked that deep into libstdc++ and I suspect that it has a lot of internal dependencies, so while you surely do not need some of the interfaces, you may or may not still depend on a big part of its internals - I do not know and did not try, but it may surprise you. You can get an idea by just linking a binary using the library against a static build of it and libstdc++ (not forgetting to strip the binary, of course), and comparing the size of the resulting executable that with the total size of a (stripped) executable dynamically linked together with the full (stripped) shared objects of the library and libstdc++.
If you find that the size difference is significant, but do not want to statically link everything, you try to reduce the size of libstdc++ by rebuilding it without some parts you know that you do not need (there are configure-time options for some parts of it, and you can also try to remove some independent objects at the final creation of libstdc++.so. There are some tools to optimize the size of libraries - search the web (I recall one from a company named MontaVista but do not see it on their web site now, there are some others too).
Other than the straightforward above, some ideas and suggestions to think of:
You mentioned that you use uClibc, which I never fiddled with myself (my experience with embedded programming is a lot more primitive, mostly involving assembly programming for the embedded processor and cross-compiling with minimal embedded libraries). I assume you checked this, and I know that uClibc is intended to be a lightweight but rather full standard C library, but do not forget that C++ code is hardly independent on the C library, and g++ and libstdc++ depend on quite some delicate things (I remember problems with libc on some proprietary Unix versions), so I would not just assume that g++ or the GNU libstdc++ actually works with uClibc without trying - I don't recall seeing it mentioned in the uClibc pages.
Also, if this is an embedded system, think of its performance, compute power, overall complexity, and timing/simplicity/solidity requirements. Take into consideration the complexity involved, and think whether using C++ and threads is appropriate in your embedded system, and if nothing else in the system uses those, whether it is worth introducing for that library. It may be, not knowing the library or system I cannot tell (again, embedded systems being such a wide range nowadays).
And in this case also, just a quick link I stumbled upon looking for uClibc -- if you are working on an embedded system, using uClibc, and want to use C++ code on it -- take a look at uClibc++. I do not know how much of the standard C++ stuff you need and it already supports, and it seems to be an ongoing project, so not clear if it is in a state good enough for you already, but assuming that your work is also under development still, it might be a good alternative to GCC's libstdc++ for your embedded work.
I think this guy explains quite well why that wouldn't make sense. C++ code that uses your shared object but a different libstdc++ would link alright, but wouldn't work.
I've got an application that loads .so files as plugins at startup, using dlopen()
The build environment is running on x86 hardware, but the application is being cross compiled for another platform.
It would be great if I could (as part of the automated build process) do a check to make sure that there aren't any unresolved symbols in a combination of the .so files and the application, without having to actually deploy the application.
Before I write a script to test symbols using the output of nm, I'm wondering if anyone knows of a utility that already does this?
edit 1: changed the description slightly - I'm not just trying to test symbols in the .so, but rather in a combination of several .so's and the application itself - ie. after the application loaded all of the .so's whether there would still be unresolved symbols.
As has been suggested in answers (thanks Martin v. Löwis and tgamblin), nm will easily identify missing symbols in a single file but won't easily identify which of those symbols has been resolved in one of the other loaded modules.
Ideally, a cross-nm tool is part of your cross-compiler suite. For example, if you build GNU binutils for cross-compilation, a cross-nm will be provided as well (along with a cross-objdump).
Could you use a recursive version of ldd for this? Someone seems to have written a script that might help. This at least tell you that all the dependency libs could be resolved, if they were specified in the .so correctly in the first place. You can guarantee that all the dependencies are referenced in the .so with linker options, and this plus recursive ldd would guarantee you no unresolved symbols.
Linkers will often have an option to make unresolved symbols in shared libraries an error, and you could use this to avoid having to check at all. For GNU ld you can just pass --no-allow-shlib-undefined and you're guaranteed that if it makes a .so, it won't have unresolved symbols. From the GNU ld docs:
--no-undefined
Report unresolved symbol references from regular object files.
This is done even if the linker is creating a non-symbolic shared
library. The switch --[no-]allow-shlib-undefined controls the
behaviour for reporting unresolved references found in shared
libraries being linked in.
--allow-shlib-undefined
--no-allow-shlib-undefined
Allows (the default) or disallows undefined symbols in shared
libraries. This switch is similar to --no-undefined except
that it determines the behaviour when the undefined symbols are
in a shared library rather than a regular object file. It does
not affect how undefined symbols in regular object files are
handled.
The reason that --allow-shlib-undefined is the default is that the
shared library being specified at link time may not be the
same as the one that is available at load time, so the symbols might
actually be resolvable at load time. Plus there are some systems,
(eg BeOS) where undefined symbols in shared libraries is normal.
(The kernel patches them at load time to select which function is most
appropriate for the current architecture. This is used for example to
dynamically select an appropriate memset function). Apparently it is
also normal for HPPA shared libraries to have undefined symbols.
If you are going to go with a post-link check, I agree with Martin that nm is probably your best bet. I usually just grep for ' U ' in the output to check for unresolved symbols, so I think it would be a pretty simple script to write.
The restrictions in nm turned out to mean that it wasn't possible to use for a comprehensive symbol checker.
In particular, nm would only list exported symbols.
However, readelf will produce a comprehensive list, along with all of the library dependencies.
Using readelf it was possible to build up a script that would:
Create a list of all of the libraries used,
Build up a list of symbols in an executable (or .so)
Build up a list of unresolved symbols - if there are any unresolved symbols at this point, there would have been an error at load time.
This is then repeated until no new libraries are found.
If this is done for the executable and all of the dlopen()ed .so files it will give a good check on unresolved dependencies that would be encountered at run time.
I have an executable on Linux that loads libfoo.so.1 (that's a SONAME) as one of its dependencies (via another shared library). It also links to another system library, which, in turn, links to a system version, libfoo.so.2. As a result, both libfoo.so.1 and libfoo.so.2 are loaded during execution, and code that was supposed to call functions from library with version 1 ends up calling (binary-incompatible) functions from a newer system library with version 2, because some symbols stay the same. The result is usually stack smashing and a subsequent segfault.
Now, the library which links against the older version is a closed-source third-party library, and I can't control what version of libfoo it compiles against. Assuming that, the only other option left is rebuilding a bunch of system libraries that currently link with libfoo.so.2 to link with libfoo.so.1.
Is there any way to avoid replacing system libraries wiith local copies that link to older libfoo? Can I load both libraries and have the code calling correct version of symbols? So I need some special symbol-level versioning?
You may be able to do some version script tricks:
http://sunsite.ualberta.ca/Documentation/Gnu/binutils-2.9.1/html_node/ld_26.html
This may require that you write a wrapper around your lib that pulls in libfoo.so.1 that exports some symbols explicitly and masks all others as local. For example:
MYSYMS {
global:
foo1;
foo2;
local:
*;
};
and use this when you link that wrapper like:
gcc -shared -Wl,--version-script,mysyms.map -o mylib wrapper.o -lfoo -L/path/to/foo.so.1
This should make libfoo.so.1's symbols local to the wrapper and not available to the main exe.
I can only come up with a work-around. Which would be to statically link a version of the "system library" that you are using. For your static build, you could make it link against the same old version as the third-party library. Given that it does not rely on the newer version...
Perhaps it is also possible to avoid these problems with not linking to the third-party library the ordinary way. Instead, your program could load it at execution time. Perhaps then it could be shadowed against the rest. But I don't know much about that.