Trying to locate function using dlfunc with RLTD_NEXT from JNI

[Konstantin Belousov]

On Wed, Nov 30, 2016 at 11:59:20AM +0100, Matthieu Volat wrote:
> Hi, a (maybe not) tricky question for people who know well the internals of the jdk on freebsd...
> 
> Trying to port a linux project that does quite a bit of introspection, begining by using malloc & co wrapper, with java binding, I found that strangely, you can use dlfunc(RTLD_NEXT, "malloc") in a non-java environment... But as soon as this code is loaded in the context of the jvm, the symbol won't be resolved.
> 
> This is maybe a bit suprising as this technique worked on Linux and I did not expect much difference here...
> 
> Small demonstration code:
> 
>   % git clone https://gist.github.com/f6993d7a48ecbab6c0c979b9a7a40912.git jni_dlfunc
>   % cd jni_dlfunc
>   % make
>   [...]
>   % ./testfoo 
>   malloc function located at 0x800ade210
>   % java -Djava.library.path=. testfoo
>   could not locate function malloc: Undefined symbol "malloc"
> 
> Would anybody have an idea of why the jvm would cause the C symbols not to be found via RTLD_NEXT and if there would be a way to workaround it (that is not to load explicitely "/lib/libc.so.7")?
> 
It does not work because libc.so is loaded before your dso in the global
dso order.  Look carefully at the description of the RTLD_NEXT constant.

Why cannot you use RTLD_DEFAULT special there ?