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binutils ld and new PT_GNU_PROPERTY segment

H.J. Lu hjl.tools@gmail.com
Fri Feb 21 03:00:00 GMT 2020 
On Thu, Feb 20, 2020 at 1:43 PM Fangrui Song <i@maskray.me> wrote:
>
> On 2020-02-19, H.J. Lu wrote:
> >On Wed, Feb 19, 2020 at 10:27 AM Fangrui Song <i@maskray.me> wrote:
> >>
> >> 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.
> >
> >It is kind of too late now.
>
> Better late than never. It is never late to fix the section type if we do intend to fix it.
>
> Loaders don't read sections => the section type change is backward compatible.
>
> On 2020-02-20, H.J. Lu wrote:
> >On Thu, Feb 20, 2020 at 1:37 AM Mark Wielaard <mark@klomp.org> wrote:
> >>
> >> Hi,
> >>
> >> On Wed, 2020-02-19 at 14:17 -0800, H.J. Lu wrote:
> >> > On Wed, Feb 19, 2020 at 1:46 PM Mark Wielaard <mark@klomp.org> wrote:
> >> > > This code isn't in the kernel yet. So either it gets changed to use the
> >> > > existing scheme with gnu property notes found through PT_NOTE to work
> >> > > with existing binaries. Then there is no need for PT_GNU_PROPERTY
> >> > > headers.
> >> > >
> >> > > Or some future kernel will start using PT_GNU_PROPERTY headers to find
> >> > > the gnu property notes. But that means it won't work with existing
> >> > > binaries that do not have that header. So there is no backwards
> >> > > compatibility anyway and we can define SHT_GNU_PROPERTY like above.
> >> > >
> >> > > So this actually seems the perfect time to make this decision.
> >> >
> >> > Binaries with .note.gnu.property section have been put into many
> >> > OS releases.  We must support them.
>
> We can teach newer assemblers to emit SHT_GNU_PROPERTY.
> Newer linkers can support both SHT_GNU_PROPERTY/SHT_NOTE .note.gnu.property
>
> At some point in the future, linkers can drop support for SHT_NOTE .note.gnu.property
> Then it will become a graceful degradation: the old SHT_NOTE object files will not be
> different from older object files without .note.gnu.property
>
> >> OK. Then it is option 1. The kernel will need to support PT_NOTE for
> >> parsing the properties, since such older binaries won't have a
> >> PT_GNU_PROPERTY program header. Then we can simply get rid of
> >> PT_GNU_PROPERTY since nobody uses it and all information is already
> >> available through the PT_NOTE segment.
> >>
> >
> >Kernel loader only checks ld.so and static executable.  Re-link them with
> >newer linker will get PT_GNU_PROPERTY.  But ld.so needs to check
> >PT_NOTE for older binaries.
>
> The current PT_GNU_PROPERTY usage is all about hints. They are "nice to have" but not
> "necessary to have". I don't see any problem teaching newer loaders to forget
> PT_NOTE, if we do think PT_GNU_PROPERTY is the way forward.
>
> The currently mixed status is annoying:
>
> glibc: PT_NOTE
> Proposed Linux kernel patch: PT_GNU_PROPERTY

Since this has been deployed on Linux, any changes should be discussed at

https://sourceware.org/ml/gnu-gabi/

-- 
H.J.
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