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.
If glibc x86 wants to deprecate copy relocations support,
just fix the compilers(*)/GNU ld. -fno-pic dynamically linked executables are
becoming rarer on modern Linux distributions,
When the toolchain support is sufficiently mature (e.g. ld has warned/errored),
add an opt-opt `LD_` style environment variable and let glibc ld.so warn, then
gradually
make it an error.
* I can fix Clang -fno-pic at any time. I haven't done that just to be
compatible with gcc -fno-pic.
• 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.
I have provided the solutions in my previous message.
This provides the capability to detect the ABI change at run-time as well as
optimize for STV_PROTECTED symbol lookup.
STV_PROTECTED symbols should not need a compiler option or a GNU PROPERTY to
work (efficiently).
As my previous message mentioned (gcc 4.1.2~now; GNU ld 2.11~now),
protected function addresses in a shared object likely never work, at
least for the past 20 years.
For protected data, x86 copy relocations did not work prior to circa 2015.
It never works on non-x86, gold, clang, or non-glibc.
And I doubt any project uses protected data given that its sole purpose is for
optimization while GCC 5 added unneeded indirection.
Ulrich Drepper did add elf/vis* tests into glibc in 2000, but they use
artificial inline asm .protected which does not reflect any reality.
GNU ld -shared for a protected symbol
* x86-64: broken direct access relocation, unneeded GLOB_DAT
* aarch64: broken direct access relocation, unneeded GLOB_DAT
* arm: unneeded GLOB_DAT for STT_OBJECT
* ppc32: unneeded GLOB_DAT for STT_OBJECT
* ppc64le: good, no GLOB_DAT
* mips64el: good, no GLOB_DAT
* riscv64: good, no GLOB_DAT
Perhaps for binutils in 2000, more ports had unneeded dynamic relocations which
made the elf/vis* tests more plausible. But the fragile support (acked by
multiple glibc maintainers, including Adhemerval/Carlos/Szabolcs) is definitely
largely irrelevant nowadays.
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.
ld-x86-64/protecte-func-* are artificial assembly which do not match the
reality.
They are cases where never work or aren't really promised to work before.
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.
See my first paragraph why a GNU PROPERTY may not be a good compatibility
solution.
