Creating a shared library from init.c and tramp.S

[lastlaugher]

At first, thanks yugr for creating this nice proejct.

In the example case, it works fine but there is an issue in my case.
I applied Implib.so to libA.so which I want for delay-loading.
I created a shared library, libB.so from libA.so.init.c and libA.tramp.S
And then I built an executable with libB.
The problem is the executable tries to load libA at strarup even though a function in libA is called after some processing is finished. Following is an error message.

$ ./executable
implib-gen: libA.so.4: failed to load library 'libA.so.4' via dlopen: libA.so.4: cannot open shared object file: No such file or directory
executable: /home/xyz/Implib.so/libA.so.4.init.c:61: void *load_library(): Assertion `0 && "Assertion in generated code"' failed.
Aborted

Is this expected result? Or could you help me delay loading works fine in my case?

[yugr]

Thank you for trying Implib.so.

First of all, there are a few caveats with your usecase (putting wrappers in a dedicated shared library, rather than into executable):

• you need to compile them with -DIMPLIB_EXPORT_SHIMS (otherwise wrapper symbols will not be exported from libB.so)
• due to bug Implib IMPLIB_EXPORT_SHIMS triggers an assertion when imported DSO has initializers referring to its public symbols. #18 this scenario may fail in presence of constructors in libA.so

As for the crash, is there a chance that there is a C++ constructor in executable which calls function from libA.so? This would cause libA.so to be loaded at startup. If that's not the case I suggest to run executable under gdb and attach a stacktrace. This would help to figure out what's going on.

[lastlaugher]

Hi yugr,

Thanks for your quick answer. First I tried the -DIMPLIB_EXPORT_SHIMS option but same thing. And there is no C++ constructor in the executable to call a function in libA. When I tried gdb, the libA is loaded in the _dl_init step even before starting main function. I've attached the call stack from gdb. Please let me know if you can find any clue from this info.

(gdb) bt
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007fffef5a37f1 in __GI_abort () at abort.c:79
#2  0x00007fffef5933fa in __assert_fail_base (fmt=0x7fffef71a6c0 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n",
    assertion=assertion@entry=0x7fffe1153730 "0 && \"Assertion in generated code\"", file=file@entry=0x7fffe1153714 "libA.so.4.init.c",
    line=line@entry=61, function=function@entry=0x7fffe11eab40 <__PRETTY_FUNCTION__.3281> "load_library") at assert.c:92
#3  0x00007fffef593472 in __GI___assert_fail (assertion=0x7fffe1153730 "0 && \"Assertion in generated code\"",
    file=0x7fffe1153714 "libA.so.4.init.c", line=61, function=0x7fffe11eab40 <__PRETTY_FUNCTION__.3281> "load_library") at assert.c:101
#4  0x00007fffe1076fb6 in load_library () from ../../../../core/library/lib/libB.so
#5  0x00007fffe107700d in load_lib () from ../../../../core/library/lib/libB.so
#6  0x00007ffff7de38d3 in call_init (env=0x7fffffffe298, argv=0x7fffffffe288, argc=1, l=<optimized out>) at dl-init.c:72
#7  _dl_init (BFD: reopening /tmp/shared.ya8vsp: No such file or directory

BFD: reopening /tmp/shared.ya8vsp: No such file or directory

BFD: reopening /tmp/shared.ya8vsp: No such file or directory

main_map=0x7ffff7ffe170, main_map@entry=<error reading variable: Can't read data for section '.eh_frame' in file '/tmp/shared.ya8vsp'>,
    argc=1, argv=0x7fffffffe288, env=0x7fffffffe298) at dl-init.c:119

[yugr]

Did you run implib-gen.py with --no-lazy-load? In that case generated code will try to load libA.so at program startup. Could you provide the

#define HAS_DLOPEN_CALLBACK ...
#define HAS_DLSYM_CALLBACK ...
#define NO_DLOPEN ...
#define LAZY_LOAD ...

snippet from your libA.so.init.c ?

[lastlaugher]

OMG you're correct. While I was trying several options, I forgot to set the lazy_load option back. Now it works like a charm in my nested shared library case. Many thanks, yugr. You saved my life :)

[yugr]

Closing for now, let me know if you have other issues.