On Thu, 2019-05-02 at 12:10 +0100, Dave Martin wrote:
> On Wed, May 01, 2019 at 02:12:17PM -0700, Yu-cheng Yu wrote:
> > An ELF file's .note.gnu.property indicates features the executable file
> > can support. For example, the property GNU_PROPERTY_X86_FEATURE_1_AND
> > indicates the file supports GNU_PROPERTY_X86_FEATURE_1_IBT and/or
> > GNU_PROPERTY_X86_FEATURE_1_SHSTK.
[...]
> A couple of questions before I look in more detail:
>
> 1) Can we rely on PT_GNU_PROPERTY being present in the phdrs to describe
> the NT_GNU_PROPERTY_TYPE_0 note? If so, we can avoid trying to parse
> irrelevant PT_NOTE segments.
Some older linkers can create multiples of NT_GNU_PROPERTY_TYPE_0. The code
scans all PT_NOTE segments to ensure there is only one NT_GNU_PROPERTY_TYPE_0.
If there are multiples, then all are considered invalid.
>
>
> 2) Are there standard types for things like the program property header?
> If not, can we add something in elf.h? We should try to coordinate with
> libc on that. Something like
>
> typedef __u32 Elf_Word;
>
> typedef struct {
> Elf_Word pr_type;
> Elf_Word pr_datasz;
> } Elf_Gnu_Prophdr;
>
> (i.e., just the header part from [1], with a more specific name -- which
> I just made up).
Yes, I will fix that.
[...]
> 3) It looks like we have to go and re-parse all the notes for every
> property requested by the arch code.
As explained above, it is necessary to scan all PT_NOTE segments. But there
should be only one NT_GNU_PROPERTY_TYPE_0 in an ELF file. Once that is found,
perhaps we can store it somewhere, or call into the arch code as you mentioned
below. I will look into that.
>
> For now there is only one property requested anyway, so this is probably
> not too bad. But could we flip things around so that we have some
> CONFIG_ARCH_WANTS_ELF_GNU_PROPERTY (say), and have the ELF core code
> call into the arch backend for each property found?
>
> The arch could provide some hook
>
> int arch_elf_has_gnu_property(const Elf_Gnu_Prophdr *prop,
> const void *data);
>
> to consume the properties as they are found.
>
> This would effectively replace the arch_setup_property() hook you
> currently have.
>
> Cheers
> ---Dave
>
> [1] https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI
