Re: RFC: ABI support for special memory area

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

On Thu, Mar 30, 2017 at 9:02 AM, Suprateeka R Hegde
<hegdesmailbox@gmail.com> wrote:
> On 03/27/17 21:45, H.J. Lu wrote:
>>
>> There is a way to support GNU_MBIND segments without the glibc changes.
>> Instead, dl_iterate_phdr
>>
>> int dl_iterate_phdr (int (*callback) (struct dl_phdr_info *info,
>>                                       size_t size, void *data),
>>                      void *data);
>>
>> is called via the .init_array section to process GNU_MBIND segments in
>> executable and shared objects:
>>
>> static int
>> callback (struct dl_phdr_info *info, size_t size, void *data)
>> {
>>   Compute the load address of the current module.
>>   if info->dlpi_addr == the load address of the current module
>>     {
>>       check ELF program headers and process GNU_MBIND segments
>>       return 1;
>>     }
>>
>>   return 0;
>> }
>>
>> static void
>> call_gnu_mbind_setup (void)
>> {
>>   dl_iterate_phdr (callback, NULL);
>> }
>>
>> static void (*init_array) (void)
>>  __attribute__ ((section (".init_array"), used))
>>  = &call_gnu_mbind_setup;
>
> This looks very ideal and perfect and matches my requirement too. Are
> you suggesting this dl_iterate_phdr(3) as the way in your proposal
> instead of the __gnu_mbind_setup?

Yes.

> Or are you suggesting that for all the implementations  that need
> different arguments (like that of my NVM) compared to
> __gnu_mbind_setup_v1, we go with this dl_iterate_phdr(3) way?

__gnu_mbind_setup_v1 is removed so that it will work with
existing C libraries with dl_iterate_phdr.

> I am OK either way.
>
> However, I am just thinking that your earlier approach --
> __gnu_mbind_setup -- is better when shared libraries with GNU_MBIND
> segments are dlopen'ed. They dont have to iterate all over again to
> reach their PHDR. Or what is the recommendation for such dlopen'ed
> libraries?

It is true that dl_iterate_phdr is called by every shared object, dlopened or
not, to locate its own PHDR.

> And this dl_iterate_phdr(3) not being part of any standards, may change
> in a totally incompatible way in the future.
>

dl_iterate_phdr isn't in any standard.  But it is in glibc.  Given that my
proposal is a GNU extension, it isn't a major issue.  Working with
existing glibc is a big plus.

-- 
H.J.