binutils ld and new PT_GNU_PROPERTY segment

[Fangrui Song]

On 2020-02-19, H.J. Lu wrote:
>On Wed, Feb 19, 2020 at 5:17 AM Mark Wielaard <mark@klomp.org> wrote:
>>
>> On Wed, 2020-02-19 at 04:28 -0800, H.J. Lu wrote:
>> > On Wed, Feb 19, 2020 at 4:02 AM Mark Wielaard <mark@klomp.org> wrote:
>> > > > https://patchwork.kernel.org/patch/11285409/
>> > > >
>> > > > It is for both x86 and arm64.
>> > >
>> > > So that is not upstream in the mainline kernel? Why can't that patch
>> > > use the existing PT_NOTE segment? That would make it compatible with
>> > > existing binaries that don't have this PT_GNU_PROPERTY program header.
>> >
>> > Kernel loader is one of motivations of PT_GNU_PROPERTY.  Kernel loader
>> > only wants to check PT_XXX.
>>
>> So they can check PT_NOTE because it provides the same information and
>> is already available in existing binaries.
>>
>
>Please take a look at glibc note.gnu.property parser.  It is very complicated to
>check for invalid .note.gnu.property sections generated by the older
>linkers with
>the new object.  Kernel loader doesn't want to do it.

One way to make things follow the spirit of https://sourceware.org/ml/gnu-gabi/2018-q4/msg00036.html

* Define SHT_GNU_PROPERTY
* Set sh_type(.note.gnu.property) to SHT_GNU_PROPERTY
* Place SHT_GNU_PROPERTY sections in a PT_GNU_PROPERTY segment

The generated PT_NOTE will not include .note.gnu.property, so the scheme is compatible with old loaders (ld.so, gdb, Linux, etc).
New loaders should interpret PT_GNU_PROPERTY, instead of PT_NOTE.
   ( https://patchwork.kernel.org/patch/11285409/ needs no change)

This way linkers can keep treating SHT_NOTE sections as opaque and apply "Rules for Linking Unrecognized Sections" (http://www.sco.com/developers/gabi/latest/ch4.sheader.html ) when combining SHT_NOTE sections. At least for lld, there will be no special rules for input SHT_NOTE sections.

I will be happy to make changes to lld and LLVM binary utilities if this
scheme reaches consensus.