Re: Define __start/__stop symbols when there is only a dynamic def

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Jan 29, 2018 at 3:32 PM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Mon, 29 Jan 2018, H.J. Lu wrote:
>
>> I asked specially if the testcase is correct:
>>
>> https://sourceware.org/ml/binutils/2018-01/msg00421.html
>
> Gah!  Sorry, I didn't recognize that you also adjusted the link line to
> remove the reference to liborphan.  Okay, with that testcase and that link
> command and no dynamic definitions of __start___verbose, then no, that's
> not a testcase that has to work (it also didn't before 2.29).  It's
> specifically the case that:
>   a) a dynamic library X exports such symbol for section S
>   b) another library or app Y linked against X also has a section S
>   c) Y doesn't contain any regular references to symbol __start_S
> then it still needs to be the case that Y exports its own variant of
> __start_S as well.  If Y doesn't contain ref_regular refs and isn't linked
> against any such X lib, then nothing needs to happen.
>
> (That's how far the old behaviour was and what is relied upon by the
> pacemaker logging mechanism.  We of course can declare that invalid,
> though it'd change IMHO useful behaviour)
>
> I now realize that the references to older mails from me might have
> confused things, so to be absolutely certain we're talking about the right
> thing.  The following it is, a two-file, two-DSO testcase:
>
> % cat lib.c
> extern int __start___verbose[];
> extern int __stop___verbose[];
> extern int foo2(void);
> int foo (void)
> {
>   static int my_var __attribute__((section("__verbose"))) = 5;
>   if (__start___verbose == __stop___verbose  // ref_regular
>       || __start___verbose[0] != 5)
>     return -1;
>   else
>     return 0;
> }
> % cat app.c
> #define _GNU_SOURCE
> #include <stdlib.h>
> #include <stdio.h>
> #include <dlfcn.h>
>
> extern int __start___verbose[];
> extern int __stop___verbose[];
> int bar (void)
> {
>   static int my_var __attribute__((section("__verbose"))) = 6;
>   int *ptr;
>   ptr = (int*) dlsym(RTLD_DEFAULT, "__start___verbose");
>   if (!ptr || *ptr != 6)
>     return -1;
>   return 0;
> }
>
> int main()
> {
>   if (bar () != 0)
>     {
>       printf("main: wrong __start___verbose\n");
>       exit(2);
>     }
>   return 0;
> }
> % cc -g   -c -o app.o app.c
> % cc -g -fPIC   -c -o lib.o lib.c
> % cc -B./ -shared -o liborphan.so lib.o -Wl,-R,.
> % cc -B./ -o app app.o liborphan.so -ldl -Wl,-R,.
> % ./app
> <nothing>

There is a reason why I have been keeping asking for a testcase.
I will update my patch with the above testcase.

Thanks.

-- 
H.J.