Re: ELF NT_GNU_PROPERTY_TYPE_0 Questions
From: Florian Weimer
Date: Thu Aug 22 2019 - 09:45:42 EST
* Dave Martin:
> Hi there,
>
> Can you clarify a couple of points about the SysV ABI Linux
> Extensions [1] for me?
>
> 1) Can there be more than one NT_GNU_PROPERTY_TYPE_0 note in a valid
> ELF file? I think the answer should be "no".
Yes, if it has been produced by a link editors which does not about
property notes. The ELF file still needs to be treated as valid, but
the note should be ignored.
> 2) Is is permissible for an ELF ET_EXEC or ET_DYN file that contains
> an NT_GNU_PROPERTY_TYPE_0 property not to have a PT_GNU_PROPERTY phdrs
> entry mapping it? Except for historical usage by RedHat (which
> apparently can be worked round in userspace) it seems reasonable for
> the answer to be "no", at least for Linux.
Using an older link editor on a CET-enabled distribution will produce
such binaries, too. The ELF file still needs to be treated as valid,
but the property date should be ignored.
> 3) Is it permissible for the PT_GNU_PROPERTY phdr (if present) to
> map anything other than precisely one NT_GNU_PROPERTY_TYPE_0
> note? I think the answer should be "no".
Correct. Additional processing logic in the link editor is needed.
> 4) Is an NT_GNU_PROPERTY_TYPE_0 note allowed to contain two or more
> properties with the same pr_type? I think the answer should be "no".
H.J. needs to answer that.
> 5) What's the rationale for sorting the properties by pr_type? I can
> see this would make it easier for the linker to merge
> NT_GNU_PROPERTY_TYPE_0 notes from different files, but I'm wondering
> whether the kernel really needs to enforce the ordering when loading
> an ELF. The kernel doesn't need to merge property lists together.
Likewise.
> 6) Do you have a view on the best way to define the Elf_Prop type in
> headers? bfd elf-bfd.h seems to have elf_property, but this doesn't
> follow the style of the public ELF headers.
We should put it into <elf.h> in glibc. We don't want to rely on UAPI
headers there because this version of <elf.h> is used in many places.
Thanks,
Florian