Re: Adding static-PIE support to binutils

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

On Tue, Aug 18, 2015 at 5:58 PM, Rich Felker <dalias@libc.org> wrote:
> On Tue, Aug 18, 2015 at 09:30:56AM -0700, H.J. Lu wrote:
>> On Tue, Aug 18, 2015 at 9:08 AM, Rich Felker <dalias@libc.org> wrote:
>> > On Tue, Aug 18, 2015 at 08:56:00AM -0700, H.J. Lu wrote:
>> >> On Mon, Aug 17, 2015 at 8:44 PM, Rich Felker <dalias@libc.org> wrote:
>> >> > On Mon, Aug 17, 2015 at 10:42:56PM -0400, Rich Felker wrote:
>> >> >> On Mon, Aug 17, 2015 at 02:19:34PM -0700, H.J. Lu wrote:
>> >> >> > On Tue, Jun 23, 2015 at 9:18 PM, Rich Felker <dalias@libc.org> wrote:
>> >> >> > > For background on the static PIE model I'm working with, see the
>> >> >> > > following post to the GCC list:
>> >> >> > >
>> >> >> > > https://gcc.gnu.org/ml/gcc/2015-06/msg00008.html
>> >> >> > >
>> >> >> > > So far, I've been prototyping static PIE support by having GCC pass
>> >> >> > > the following options to ld instead of -static -pie:
>> >> >> > >
>> >> >> > >         -static -shared -Bsymbolic
>> >> >> > >
>> >> >> > > This partly works, but since ld does not know it's producing a main
>> >> >> > > executable, it misses important details, including the ability to link
>> >> >> > > initial-exec and local-exec model TLS code correctly, as well as
>> >> >> > > various linking optimizations. So I think the right way forward is
>> >> >> > > making ld accept -static and -pie together to do the right thing.
>> >> >> > >
>> >> >> > > In elflink.c, _bfd_elf_link_create_dynamic_sections assumes that
>> >> >> > > executables should always have a .interp section.
>> >> >> > > bfd_elf_size_dynamic_sections asserts this assumption again, and the
>> >> >> > > individual elf??-*.c files also do so in *_elf_size_dynamic_sections
>> >> >> > > where they set a default interpreter. (Is this even useful? Most of
>> >> >> > > the names are out of touch with reality, and GCC always passes an
>> >> >> > > explicit -dynamic-linker anyway, so I think this code should just be
>> >> >> > > removed.)
>> >> >> > >
>> >> >> > > Now I have a working prototype by changing the info->executable
>> >> >> > > condition to info->executable && info->dynamic, and having lexsup.c
>> >> >> > > store the value of input_flags.dynamic in link_info.dynamic after
>> >> >> > > processing the command line, but I'm not sure if this is the right
>> >> >> > > approach.
>> >> >> >
>> >> >> > It is OK to use -static/-Bstatic/-non_shared with -shared and -pie.
>> >> >> > I think you want --no-dynamic-linker.
>> >> >>
>> >> >> I see two overall approaches to making the option to omit .interp:
>> >> >>
>> >> >> 1. In elflink.c, make the creation of the .interp section conditional
>> >> >>    on a new field in link_info.
>> >> >>
>> >> >> 2. In ld code (ldlang.c? elf32.em?), check the command line option and
>> >> >>    remove the .interp section before it can be processed.
>> >> >>
>> >> >> I think option 1 is a lot cleaner, but it's also going to be a lot
>> >> >> more invasive, because every single target arch (elf32-*.c and
>> >> >> elf64-*.c) has its own ASSERT that the .interp section exists. These
>> >> >> would also need to be updated to either check the new field in
>> >> >> link_info, or to replace the ASSERT with a conditional.
>> >> >>
>> >> >> Before I spend a lot of time implementing one or the other, do you
>> >> >> have any feelings on which way would be appropriate?
>> >> >
>> >> > I went ahead and did option 1 modulo all the target code except sh
>> >> > which is where I'm testing it. My work-in-progress patch is attached.
>> >> > This is obviously not ready to submit but I would appreciate any
>> >> > feedback that's possible at this stage.
>> >>
>> >> + case OPTION_NO_DYNAMIC_LINKER:
>> >> +  command_line.interpreter = NULL;
>> >> +  link_info.nointerp = 1;
>> >>
>> >> No need to clear command_line.interpreter and please add a simple
>> >> testcase to verify it works correctly.
>> >
>> > OK. Do I also need to update it to be against the new output_type
>> > stuff you just committed?
>> >
>>
>> That is a good idea.
>
> I've updated the patch to cover the changes needed for all the
> elf??-*.c target files (lots of code duplication already there), skip
> the clearing of command_line.interpreter, and based it on current git
> master with your output_type changes.
>
> I haven't done a test case yet -- I looked briefly but couldn't find
> documentation on how to add one. Is there a guide or template I should
> look at? And do I need to open a BZ issue for the feature request, or
> can non-bug changes like this skip BZ?
>

I don't think a new command line option is needed.  You add
a new bit, nointerp_set:

1. For -static:  if nointerp_set is 0; then nointerp = 1.
2. For -Bdynamic"  nointerlp_set = 1.

-- 
H.J.