Re: [llvm-dev] RFC: Add GNU_PROPERTY_UINT32_AND_XXX/GNU_PROPERTY_UINT32_OR_XXX

[Fangrui Song <maskray@google.com>]

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@gmail.com> wrote:
>>
>> On Wed, Jan 13, 2021 at 9:06 AM H.J. Lu <hjl.tools@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.