Excellent post, should turn it into a blog.

Kind regards,

        Peter Kriens

On 29 apr 2011, at 13:39, Holger Hoffstätte wrote:

> Sorry for writing so much but you asked for it.. :-)
> On 28.04.2011 20:08, duncan wrote:
>> We are accessing native libraries from Java. This works outside of
>> OSGi and in OSGi on Windows but not on Linux probably because the dll
>> and so linking behave differently.
> Yes and no. They work differently, but you must understand why.
> Instead of randomly "successful" expriments here is what happens & why,
> and what you can do to load native code from bundles regardless of platform.
> First off: the Bundle-NativeCode capabilitiy/idea is absolutely
> fantastic and one of the most underrated & unknown features of OSGi. It
> works well, but is naturally affected by the capabilities of the
> underlying OS and Java's behaviour. Naturally this area is harder since
> "write once, run anywhere" does not apply any longer when you venture
> outside the JVM. Nobody just walks into native land.
> I'll skip the case where you only have a single library, since that is
> easy and not a problem on either platform. Just link all your JNI stubs
> and whatever they call into one big dll/so and bingo; works everywhere.
> As you have learned this is not so easy any more when you have two or
> more libraries with dependencies between them. Since loading of
> dependencies between native libraries works differently on different
> OSes, there is nothing OSGi itself can do here; but understanding what
> exactly happens will allow us to handle the differences in a transparent
> manner.
> First off let's understand what happens when Java loads a dll/so.
> Regardless of the platform-specific call the library is loaded, and -
> before the call returns - the OS will notice missing symbols (e.g.
> functions defined in a dependent library) and start the
> platform-dependent search for dependencies to satisfies the missing bits.
> Windows will look in the current working directory of the process
> (usually useless for OSGi) and then %PATH%, Linux will inspect
> LD_LIBRARY_PATH ([1]) and then consult the ld.so cache, usually located
> in /etc/ld.so.cache. This cache contains a cached representation of the
> system-wide search paths usually declared in /etc/ld.so.conf. This can
> be updated by e.g. an installer or system management tool; see man ldconfig.
> All of these mechanisms are unfortunately not too useful for OSGi, since
> two or more libraries contained in & unpacked from a bundle are sitting
> somewhere in a bundle cache - a private directory normally not visible
> to the OS. So what can you do to trick the native platform loading into
> doing the right thing?
> Let's start with Windows since that is ironically the easiest case. As
> some other posters have indicated you can indeed pre-load dependencies
> in their correct order and then load your JNI library. The preloading
> makes sure that the last loaded dependency has no missing symbols, and
> the native search machinery does not kick in - problem averted.
> Then you try this on Linux or OSX and realize that it Simply Does Not
> Work (sorry Richard :). Why?
> The POSIX/Unix/Linux system calls used for loading, accessing and
> closing shared objects are called dlopen(), dlsym() and dlclose(). If
> you look at dlopen(), it takes a filename (the library to load) and a
> magic flag. The way the JVM calls dlopen() is the reason why preloading
> does not work: the flag is by platform-default set to RTLD_LOCAL, which
> does NOT expose loaded symbols to subsequently loaded libraries
> (contrary to Windows, where loaded symbols become visible).
> In other words: manual preloading of dependencies on Linux fixes exactly
> nothing! Whether this is just an unfortunate default, a bug or
> deliberate is a different, complicated discussion about isolation,
> security, stability etc. but ultimately does not matter since we cannot
> influence it.
> Bummer. So what now, give up? Bah!
> If preloading does not work then maybe we can make the OS do the right
> thing instead. ld.so - the dynamic loader used for resolving library
> dependencies - looks at a loaded object and obviously also needs to know
> what else to load in case there are any declared dependencies. As it
> turns out shared libraries can have a search path embedded and *this
> path can be manipulated*. Quote from the ld.so man page:
>   $ORIGIN and rpath
>       ld.so  understands  the  string  $ORIGIN  (or  equivalently
>       ${ORIGIN}) in an rpath specification (DT_RPATH or DT_RUNPATH) to
>       mean the directory containing the application executable.
>       Thus, an application located in somedir/app could be compiled
>       with gcc -Wl,-rpath,'$ORIGIN/../lib' so that it finds an
>       associated shared library in somedir/lib no matter where
>       somedir is located in the directory hierarchy. [..]
> Awesomesauce! This means that you can build your native libraries with
> e.g. -rpath,'$ORIGIN' and loading one dependency (for example your JNI
> stubs) will now correctly find any dependencies from the same directory.
> Win!
> But what if you don't have the source to the dependencies? Or even worse
> you never learned how to compile C code?
> Well, old people^h^h^h^h^h^h^h enterprising individuals like me would
> probably just bring out the old hex editor, but there is a more
> civilized way: meet patchelf [2]. For anyone who wants to dive deeper
> you can also look at [3]; I managed to make the author's "rpath" tool
> build & work on Linux with just a few modifications.
> patchelf is easy to build and allows you to inspect & modify the runpath
> of any library. For an example let's find a library that has dependencies:
> (readelf is part of binutils, which you should have installed already)
> $readelf -d /usr/lib/libpcrecpp.so.0.0.0
> Dynamic section at offset 0x8dd4 contains 28 entries:
>  Tag        Type                  Name/Value
> 0x00000001 (NEEDED)              Shared library: [libpcre.so.0]
> 0x00000001 (NEEDED)              Shared library:[libstdc++.so.6]
> 0x00000001 (NEEDED)              Shared library: [libm.so.6]
> 0x00000001 (NEEDED)              Shared library: [libc.so.6]
> 0x00000001 (NEEDED)              Shared library: [libgcc_s.so.1]
> 0x0000000e (SONAME)              Library soname: [libpcrecpp.so.0]
> 0x0000000c (INIT)                0x2af8
> That should do: the C++ wrapper for pcre needs the core C pcre library
> as dependency. Let's modify it!
> $cp /usr/lib/libpcrecpp.so.0.0.0 .
> $patchelf --set-rpath '.:$ORIGIN' libpcrecpp.so.0.0.0
> $patchelf --print-rpath libpcrecpp.so.0.0.0
> And indeed:
> holger>readelf -d libpcrecpp.so.0.0.0
> Dynamic section at offset 0xa100 contains 29 entries:
>  Tag        Type                  Name/Value
> 0x0000001d (RUNPATH)             Library runpath: [.:$ORIGIN]
> 0x00000001 (NEEDED)              Shared library: [libpcre.so.0]
> [..]
> This means that loading this library would make the ld.so loader look
> for dependencies first in the current directory of the process (which
> btw is a questionable practice, mostly for security reasons), and then
> in the directory where the originating library is located..which could
> be the completely unknown and anonymous bundle cache of the OSGi
> runtime. Win!
> So you modify all your .so libraries, package them into the bundle
> and..OH NOES! It STILL does not work! Depending on the OSGi runtime you
> still get errors about the native dependency not being found, despite
> "You didn't think it would be that easy, did you? Silly Rabbit."
> You see, what happens is that the OSGi runtime is at liberty to unpack
> resources from a bundle lazily. This means that simply loading a
> toplevel dependency is not enough, as any other bundle-included
> dependencies may not yet have been unpacked, causing the native loader
> to fail. The fix is easy: simply *pretend* to preload all libraries
> (just like on Windows), but ignore any UnsatisfiedLinkErrors - and then
> load the JNI stubs.
> Et voilà: angels will sing, and your bundle will work.
> To make this robust don't just sprinkle the System.loadLibrary() calls
> throughout your classes; make a central Initializer class (bound to the
> bundle's Activator or not..your choice) and refer to it. Then in that
> activator you can have differetn loading strategies for Windows
> (preloading nicely in-order), Linux (fake preloading) OSX (same as
> Linux) or any other platform. It's also a good idea not to refer to the
> classes in this bundle as "library" (i.e. passive code); make the native
> code a service and properly track it from client bundles.
> I hope this answers all your questions and gives you more rope to hang
> yourself than you asked for. If not..just ask. :-)
> Holger
> [1] http://blogs.sun.com/rie/entr /tt_ld_library_path_tt
> [2] http://nixos.org/patchelf.html
> [3] http://blogs.sun.com/ali/entry/changing_elf_runpaths
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