Re: Adding static-PIE support to binutils

[Rich Felker <dalias at libc dot org>]

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?

Rich