[llvm-dev] RFC: Add GNU_PROPERTY_UINT32_AND_XXX/GNU_PROPERTY_UINT32_OR_XXX

Thu Jun 17 16:01:12 PDT 2021

On Thu, Jun 17, 2021 at 1:25 PM Fāng-ruì Sòng <maskray at google.com> wrote:
>
> On Thu, Jun 17, 2021 at 12:46 PM H.J. Lu <hjl.tools at gmail.com> wrote:
> >
> > On Thu, Jun 17, 2021 at 12:38 PM Fangrui Song <maskray at google.com> wrote:
> > >
> > > On 2021-06-17, H.J. Lu via llvm-dev wrote:
> > > >On Thu, Jan 21, 2021 at 7:02 AM H.J. Lu <hjl.tools at gmail.com> wrote:
> > > >>
> > > >> On Wed, Jan 13, 2021 at 9:06 AM H.J. Lu <hjl.tools at gmail.com> wrote:
> > > >> >
> > > >> > 1. GNU_PROPERTY_UINT32_AND_LO..GNU_PROPERTY_UINT32_AND_HI
> > > >> >
> > > >> >  #define GNU_PROPERTY_UINT32_AND_LO 0xb0000000
> > > >> >  #define GNU_PROPERTY_UINT32_AND_HI 0xb0007fff
> > > >> >
> > > >> > A bit in the output pr_data field is set only if it is set in all
> > > >> > relocatable input pr_data fields.  If all bits in the the output
> > > >> > pr_data field are zero, this property should be removed from output.
> > > >> >
> > > >> > If the bit is 1, all input relocatables have the feature.  If the
> > > >> > bit is 0 or the property is missing, the info is unknown.
> > >
> > > How to use AND in practice?
> > > Are you going to add .note.gnu.property to all of crt1.o crti.o
> > > crtbegin.o crtend.o crtn.o and miscellaneous libc_nonshared.a object
> > > files written in assembly?
> > >
> > > >> > 2. GNU_PROPERTY_UINT32_OR_LO..GNU_PROPERTY_UINT32_OR_HI
> > > >> >
> > > >> >  #define GNU_PROPERTY_UINT32_OR_LO 0xb0008000
> > > >> >  #define GNU_PROPERTY_UINT32_OR_HI 0xb000ffff
> > > >> >
> > > >> > A bit in the output pr_data field is set if it is set in any
> > > >> > relocatable input pr_data fields. If all bits in the the output
> > > >> > pr_data field are zero, this property should be removed from output.
> > > >> >
> > > >> > If the bit is 1, some input relocatables have the feature.  If the
> > > >> > bit is 0 or the property is missing, the info is unknown.
> > > >> >
> > > >> > The PDF is at
> > > >> >
> > > >> > https://gitlab.com/x86-psABIs/Linux-ABI/-/wikis/uploads/0690db0a3b7e5d8a44e0271a4be54aa7/linux-gABI-and-or-2021-01-13.pdf
> > > >> >
> > > >> > --
> > > >> > H.J.
> > > >>
> > > >> Here is the binutils patch to implement it.
> > > >>
> > > >
> > > >If there are no objections, I will check it in tomorrow.
> > >
> > > If the use case is just ELF_RTYPE_CLASS_EXTERN_PROTECTED_DATA, it'd be
> > > very kind of you if you can collect more use cases before generalizing
> > > this into a non-arch-specific GNU PROPERTY.
> > >
> > > The "copy relocations on protected data symbols" thing is x86 specific
> > > and only applies with gcc+GNU ld+glibc.
> > > Non-x86 architectures don't have this thing.
> > > gold doesn't have this thing.
> > > clang doesn't have this thing.
> >
> > It will be used to remove copy relocation and implement canonical function
> > pointers, which will benefit protected data and function.
>
> The action items in
> https://gitlab.com/x86-psABIs/x86-64-ABI/-/issues/8#note_593822281
> can be applied without a GNU PROPERTY.
>
> If we want to enforce the link-time check that a shared object is no longer
> compatible with copy relocations, just make the shared object's non-weak
> definitions protected, and add a GNU ld diagnostic like gold
> (https://sourceware.org/bugzilla/show_bug.cgi?id=19823)
>
> ---
>
> For functions,
>
> On x86-64, gcc -fpic has been using  leaq    addr()(%rip), %rax since at least
> 4.1.2 (oldest gcc I can find on godbolt):
>
>   __attribute__((visibility("protected")))
>   void *addr() { return (void*)addr; }
>
>   // a protected non-definition declaration is the same.
>
>   // while asm(".protected addr") can use GOT, it is super rare if ever exists
>   // outside glibc elf/vis*.c
>
> I have checked all of binutils 2.11, 2.16, 2.20, 2.24, 2.35. The have
> the same diagnostic:
>
>   relocation R_X86_64_PC32 against protected function `addr' can not
> be used when making a shared object
>
> I think we can assert that taking the address of a protected function
> never works with GNU ld.
> So no compatibility concern.
> Fixing it (https://sourceware.org/pipermail/binutils/2021-June/116985.html)
> doesn't need any GNU PROPERTY.
>
> ---
>
> For variables, if an object file/archive member does not have GNU PROPERTY, do
> you consider it incompatible with "single global definition"? That is why I
> mentioned crt1.o crti.o crtbegin.o crtend.o crtn.o and libc_nonshared.a members
> written in assembly.
>
> If you consider such an object compatible with "single global definition", I
> don't see why a GNU PROPERTY is needed.
>
> If you consider such an object incompatible with "single global definition", I
> don't see how "single global definition" benefits can be claimed giving so many
> prebuilt object files without GNU PROPERTY.

Please see the slides in

https://gitlab.com/x86-psABIs/x86-64-ABI/-/issues/8

which includes

Dynamic Linker for Single Global Definition
• Check the single global definition marker on all components, the executable
and its dependency shared libraries.
• Issue an error/warning if the marker is not consistent on all components.
• Disallow copy relocation against definition in the shared library with the
marker.
• For systems without function descriptor:
• Disallow function pointer reference in executable without the marker to the
definition with the STV_PROTECTED visibility in a shared library with
the marker.
• Use the address of the function body as function pointer on functions with the
STV_PROTECTED visibility, which are defined in shared libraries with the marker.

This provides the capability to detect the ABI change at run-time as well as
optimize for STV_PROTECTED symbol lookup.

My linker implementation is at

https://gitlab.com/x86-binutils/binutils-gdb/-/tree/users/hjl/property/master

I will implement the dynamic linker change.

> If we still want "absolutely no copy relocation for -fno-pic", just use GOT for
> default visibility external data access
> (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112)
> Some architectures may not like it (i386/ppc32), just leave them behind.
> Modern architectures can do it. When things get matured, add a ld warning,
> then add a ld.so warning. When things get more matured, change the warnings to
> errors.
>
> Such changes should use a mechanism similar to glibc LD_DYNAMIC_WEAK (weak can
> preempt global) and Solaris LD_BREADTH (breadth-first order based dependency
> order) and LD_NODIRECT (direct bindings). At some point, introduce a behavior
> change.  I don't think how an explicit marker can improve the compatibility
> story. The conceived compatibility issues likely don't really exist for

The compatibility issue does exist.  Please see the linker tests I added.

> functions. For copy relocations, I think we may need to wait an extended period
> of time.

That is what the single global definition marker is used for.

-- 
H.J.