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

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:

quoted

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