[llvm-dev] RFC: Add GNU_PROPERTY_UINT32_AND_XXX/GNU_PROPERTY_UINT32_OR_XXX

[Fāng-ruì Sòng via llvm-dev]

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.

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