Re: [PATCH] binfmt_elf: Extract .note.gnu.property from an ELF file

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

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