On Thu, Jun 17, 2021 at 5:06 PM Fāng-ruì Sòng <mask...@google.com> wrote: > > On 2021-06-17, H.J. Lu wrote: > >On Thu, Jun 17, 2021 at 1:25 PM Fāng-ruì Sòng <mask...@google.com> wrote: > >> > >> On Thu, Jun 17, 2021 at 12:46 PM H.J. Lu <hjl.to...@gmail.com> wrote: > >> > > >> > On Thu, Jun 17, 2021 at 12:38 PM Fangrui Song <mask...@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.to...@gmail.com> wrote: > >> > > >> > >> > > >> On Wed, Jan 13, 2021 at 9:06 AM H.J. Lu <hjl.to...@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. • Disallow copy relocation against definition with the STV_PROTECTED visibility in the shared library with the marker. • 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. • 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.
