[PATCH v3 2/2] elf: Add GNU_PROPERTY_1_NEEDED check

[H.J. Lu]

On Fri, Jun 25, 2021 at 7:10 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu:
>
> > If GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS is set on any input
> > relocatable files:
> >
> > 1. Don't generate copy relocations.
> > 2. Turn off extern_protected_data since it implies
> > GNU_PROPERTY_NO_COPY_ON_PROTECTED.
> > 3. Treate reference to protected symbols with indirect external access
> > as local.
> > 4. Set GNU_PROPERTY_1_NEEDED_INDIRECT_EXTERN_ACCESS on output.
> > 5. When generating executable, clear this bit when there are non-GOT or
> > non-PLT relocations in input relocatable files without the bit set.
> > 6. Add -z [no]indirect-extern-access to control indirect external access.
>
> Should the property be used just for error checking?  We would flip the
> default unconditionally.  Such a behavioral change simply based on some
> input file is quite surprising.

The property is used to to allow compiling sources with
-fno-direct-extern-access
by pieces.  When creating a shared library, if one input relocatable file
is compiled with -fno-direct-extern-access, linker will bind all protected
symbols locally before seeing ALL relocations against them in different
input relocatables files.

> For (4), I think we need to set a different flag (or perhaps even
> flags), and be really careful about what we do.  I think an output file
> that is an executable will never require indirect-extern-access, but it

What did you mean by that?  We need to compile executable with
-fno-direct-extern-access for the whole scheme to work.

> can be incompatible with indirect-extern-access objects at run time.
> Shared objects as output files may themselves depend on
> indirect-extern-access objects at run time.  Ideally, markup would be
> applied to the relocations that are affected by the changes in the ABI.

That is what my glibc changes do:

$ ./elf/tst-protected1a
./elf/tst-protected1a: protected1:
/export/build/gnu/tools-build/glibc-gitlab/build-x86_64-linux/elf/tst-protected1moda.so:
copy relocation against non-copyable protected symbol
$ readelf -r ./elf/tst-protected1a | grep COPY
0000004071d8  004300000005 R_X86_64_COPY     00000000004071d8 protected1 + 0
0000004071dc  004600000005 R_X86_64_COPY     00000000004071dc protected3 + 0

This error happens only if there is a copy relocation against protected symbol
definition compiled with -fno-direct-extern-access.

> Thanks,
> Florian
>


-- 
H.J.