[llvm-dev] RFC: Add GNU_PROPERTY_UINT32_AND_XXX/GNU_PROPERTY_UINT32_OR_XXX

Thu Jun 17 19:40:54 PDT 2021

On Thu, Jun 17, 2021 at 5:49 PM Fāng-ruì Sòng <maskray at google.com> wrote:
>
> On Thu, Jun 17, 2021 at 5:24 PM H.J. Lu <hjl.tools at gmail.com> wrote:
> >
> > On Thu, Jun 17, 2021 at 5:06 PM Fāng-ruì Sòng <maskray at google.com> wrote:
> > >
> > > On 2021-06-17, H.J. Lu wrote:
> > > >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.
> > >
> > > This is not appealing from a compatibility point of view.
> > > It is common that a system has mixed shared objects:
> > >
> > > -fsingle-global-definition => a.so (marker value 1)
> > > no -fsingle-global-definition => b.so (marker value 0 or no marker)
> > > Issuing a warning will be annoying.
> > >
> >
> > I updated my proposal to
> >
> > Dynamic Linker for Single Global Definition
> > • Check the single global definition marker on all components, the executable
> > and its dependency shared libraries.
>
> I find that I forgot (in so many of my previous messages) to mention
> that the name "single global definition" may give a false impression.
> For example, a dynamic STV_DEFAULT STB_WEAK/STB_GLOBAL symbol defined
> in a shared object can still be interposed.
>
> > • Disallow copy relocation against definition with the STV_PROTECTED
> > visibility in the shared library with the marker.
>
> If this is for GNU ld x86 only, I'm fine with it:)
>
> gold and ld.lld just emit an error unconditionally. I think non-x86
> GNU ld ports which never support "copy relocations on protected data
> symbols" may want to make the diagnostic unconditional as well.
> Well, while (Michael Matz and ) I think compatibility check for "copy
> relocations on protected data symbols" is over-engineering (and
> Alan/Cary think it was a mistake), if you still want to add it, it is
> fine for me...
> For Clang, I hope we will not emit such a property, because Clang
> never supports the "copy relocations on protected data symbols"
> scheme.

The issue is that libfoo.so used in link-time can be different from
libfoo.so at run-time.   The symbol, foobar, in libfoo.so at link-time
has the default visibility.  But foobar in libfoo.so at run-time can be
protected.  ld.so should detect such cases which can lead to run-time
failures.

> > • For systems without function descriptor:
> > • Disallow non-GOT function pointer reference in executable without
> > the marker to the
> > definition with the STV_PROTECTED visibility in a shared library with
> > the marker.
>
> I think this can be unconditional, because the "pointer equality for
> STV_PROTECTED function address in -shared" case hasn't been working
> for GNU ld for at least 20 years...
> Many ports don't even produce a dynamic relocation.

True.  But see above.   You may not care about such use cases.  But I believe
that ld.so should not knowingly and silently allow such run-time
failure to happen.

> I don't mind if you add it just for symmetry, but it just feels unneeded.
>
> > • 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.
>
>
> >
> > --
> > H.J.

-- 
H.J.