[Bug dynamic-link/30127] [rfe]: enable ld audit at run-time

["stsp at users dot sourceforge.net" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30127

--- Comment #7 from Stas Sergeev <stsp at users dot sourceforge.net> ---
(In reply to Florian Weimer from comment #6)
> There is a problem with copy relocations against _r_debug. If you try to
> access _r_debug programmatically, try to build with -fPIC.

Oh wow, I was using -fpie, but indeed adding also -fpic to executable
fixes _r_debug! Very confusing...

Still no luck though.
To get the handle, I convert the struct link_map ptr to 

struct link_map_extended {
    /* These first few members are part of the protocol with the debugger.
       This is the same format used in SVR4.  */

    ElfW(Addr) l_addr;          /* Difference between the address in the ELF
                                   file and the addresses in memory.  */
    char *l_name;               /* Absolute file name object was found in.  */
    ElfW(Dyn) *l_ld;            /* Dynamic section of the shared object.  */
    struct link_map *l_next, *l_prev; /* Chain of loaded objects.  */

    /* All following members are internal to the dynamic linker.
       They may change without notice.  */

    /* This is an element which is only ever different from a pointer to
       the very same copy of this type for ld.so when it is used in more
       than one namespace.  */
    struct link_map *l_real;

    /* Number of the namespace this link map belongs to.  */
    Lmid_t l_ns;
};

I can see that the ns member seems correct:
it is 0 for first namespace and 1 for audit namespace.
But this fails:

void *h = dlmopen(((struct link_map_extended *)l1)->l_ns, l1->l_name, RTLD_LAZY
| RTLD_LOCAL | RTLD_NOLOAD);

... with this dlerror():
./tst-auditmod18.so: invalid target namespace in dlmopen(): Invalid argument


So I still can't get a handle. :(
Plus I use undocumented struct link_map_extended, which
I probably shouldn't do.
So what is to try?

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