Re: protected __start_section and __stop_section symbols

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

On Mon, Feb 5, 2018 at 4:56 PM, Cimbali <me@cimba.li> wrote:
> Hi HJ Michael and Alan, and thanks for your replies.
>
> On 05/02/18 22:28, H.J. Lu wrote:
>>
>> __start_XXX and __stop_XXX symbols are defined for section XXX within
>> the executable or share objects.  You can create a shared object which
>> contains section XXX to provide the default __start_XXX and __stop_XXX
>> if they aren't defined in the executable.
>
> Yes that's the behaviour I want, however it does *not* work anymore. The
> shared object symbols do not act as "default", in the sense that the values
> from the shared object are always used. Even when they are defined in the
> executable.
>
> Just to check that we mean the same thing, here's a simple test case:
>
> ==> bar.c <==
> #include <stdio.h>
> typedef void (*fptr)();
>
> void pre_init() { printf("OK -- run me from library constructor\n"); }
> fptr section_fptrlist __attribute__((section("fptrlist"))) = (fptr)pre_init;
>
> int main() { return 0; }
>
> ==> foo.c <==
> #include <stdio.h>
> typedef void (*fptr)();
>
> void dummy() { printf("NO -- I should be overriden by prog_function\n"); }
>
> fptr section_fptrlist __attribute__((section("fptrlist"))) = (fptr)dummy;
> extern fptr __start_fptrlist, __stop_fptrlist;
>
> void __attribute__((constructor)) setup()
> {
>     // setup library: try to call pre_init()
>     for (fptr *f = &__start_fptrlist; f != &__stop_fptrlist; f++)
>         (*f)();
> }
>
> $ gcc -g -O0    -fPIC -shared foo.c -o libfoo.so
> $ gcc -g -O0    bar.c -L. -lfoo -o linked
> $ LD_LIBRARY_PATH=. ./linked
> NO -- I should be overriden by prog_function
>
> Until commit cbd0eecf2, running this code resulted in "OK -- run me from
> library constructor".
>

This is not what I meant.  The shared object, which provide sthe default
__start_XXX and __stop_XXX, shouldn't care which __start_XXX and
__stop_XXX are used inside itself.  If you don't have one, you can add one.
Or you can use weak symbols to reference __start_XXX and __stop_XXX
in the executable.

-- 
H.J.