Re: Adding static-PIE support to binutils

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

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.

H.J.

-- 
H.J.