Re: RFC: Add DT_GNU_DEBUG

[Florian Weimer via Libc-alpha <libc-alpha@sourceware.org>]

* Daniel Walker:

> On Mon, Aug 02, 2021 at 06:10:55AM -0700, H.J. Lu wrote:
>> On Sun, Aug 1, 2021 at 10:22 PM Florian Weimer <fweimer@redhat.com> wrote:
>> >
>> > * H. J. Lu:
>> >
>> > > On Wed, Jul 28, 2021 at 1:04 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>> > >> > Do you want to drive this, or should I ? It looks like you know the people
>> > >> > involved better than I do.
>> > >> >
>> > >>
>> > >> https://groups.google.com/g/generic-abi/c/1ngxmSwrafc
>> > >>
>> > >
>> > > I don't think the gABI community is interested in a new debug dynamic
>> > > tag.  I propose DT_GNU_DEBUG:
>> > >
>> > > #define DT_GNU_DEBUG   0x6ffffff8
>> > >
>> > > for the address of a pointer which will be filled by the dynamic
>> > > linker.  Linker should
>> > > add the DT_GNU_DEBUG entry to executable's dynamic section.
>> > >
>> > > BTW, we have a choice.  DT_GNU_DEBUG can be readonly or readonly after
>> > > relocation, like DT_DEBUG.
>> >
>> > What about adding DT_DEBUG_SIZE, specifying the size of the data pointed
>> > to by DT_DEBUG?
>> 
>> It works if we don't need to support static executables.
>> 
>> > Perhaps the gABI folks are interested in that, too?  I think it's worth
>> > a try.  If the answer is “no”, we can still add DT_GNU_DEBUG_SIZE to the
>> > GNU ABI.
>> 
>> I did.  I didn't get any feedback.
>
> So no feedback equal "not interested" ?

I think the issue is that the struct already has a version field.

It's only a problem for us because we have architectures with copy
relocations *and* have exposed _r_debug as a public symbol.  However, we
can do what we did for _sys_errlist in the past (new symbol versions for
each size change), so it's not a real blocker.

I suppose the easiest way forward would be to grow _r_debug this way,
bump the version field past Solaris' version, add the Solaris members
(possibly with dummy values), and add our own new stuff afterwards.

Thanks,
Florian