H.J. Lu via llvm-dev
2021-Jun-17 19:45 UTC
[llvm-dev] RFC: Add GNU_PROPERTY_UINT32_AND_XXX/GNU_PROPERTY_UINT32_OR_XXX
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. -- H.J.
Fāng-ruì Sòng via llvm-dev
2021-Jun-17 20:25 UTC
[llvm-dev] RFC: Add GNU_PROPERTY_UINT32_AND_XXX/GNU_PROPERTY_UINT32_OR_XXX
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.