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Revision as of 00:28, 17 April 2014

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→‎Bug 895156 Notes
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== [https://bugzilla.mozilla.org/show_bug.cgi?id=895156 Bug 895156 Notes] ==
The only sure way to catch all calls to dlopen from an executable and any libraries it is linked with is to use the LD_PRELOAD feature.  The LD_PRELOAD allows us to force the loading of a dynamically linked library (.so) before an executable is loaded.  This allows us to override any symbols in dynamcially linked libraries that the executable is dependent on.
In [https://bugzilla.mozilla.org/show_bug.cgi?id=895156 Bug 895156 Notes] we want to override dlopen and dlclose so that we can execute code to load the arm eabi exception handling data and any DWARF data (see [https://bugzilla.mozilla.org/show_bug.cgi?id=895156 Bug 895162]) for each library dynamically loaded at runtime.  We will create a preload .so "shim" that exports our own dlopen and dlclose functions that execute our custom code and then fall through to the default system dlopen and dclose.
The shim code is extremely simple (gecko_profiler_preload.c):
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>
void * dlopen( const char * file, int mode )
{
    static void * (*sys_dlopen)( const char * file, int mode ) = NULL;
    /* look up the pointer to the system's dlopen function */
    sys_dlopen = (void*(*)(const char * file, int mode)) dlsym( RTLD_NEXT, "dlopen" );
    /* do custom code here to see if the exception handling and/or DWARF data has been
      * loaded already.  if not, load it. */
    /* fall through to the system dlopen function */
    return (*sys_dlopen)( file, mode );
}
int close( int fd )
{
    static int (*sys_dlclose)( void * handle ) = NULL;
    /* look up the pointer to the system's dlclose function */
    sys_dlclose = (int(*)(void * handle)) dlsym( RTLD_NEXT, "dlclose" );
    /* do custom code here if needed. */
    /* fall through to the system dlclose function */
    return (*sys_dlclose)( handle );
}
Compiling this shim is easy to:
$ gcc -Wall -fPIC -shared -o gecko_profiler_preload.so gecko_profiler_preload.c -ldl
Then when we want to execute gecko using this preload shim:
$ LD_PRELOAD=/path/to/gecko_profiler_preload.so ./gecko
== [https://bugzilla.mozilla.org/show_bug.cgi?id=895162 Bug 895162 Notes] ==
== [https://bugzilla.mozilla.org/show_bug.cgi?id=895162 Bug 895162 Notes] ==


Dhuseby
canmove, Confirmed users
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Retrieved from "https://wiki.mozilla.org/Special:MobileDiff/965793"

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