Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

Sami Tolvanen
2022-02-14T21:38:42Z

On Fri, Feb 11, 2022 at 5:38 AM Peter Zijlstra <peterz@infradead.org> wrote:
> I think we'll end up with something related to KCFI, but with distinct
> differences:
>
> - 32bit immediates for smaller code

Sure, I don't see issues with that. Based on a quick test with
defconfig, this reduces vmlinux size by 0.30%.

> - __kcfi_check_fail() is out for smaller code

I'm fine with adding a trap mode that's used by default, but having
more helpful diagnostics when something fails is useful even in
production systems in my experience. This change results in a vmlinux
that's another 0.92% smaller.

> Which then yields:
>
> caller:
> cmpl $0xdeadbeef, -0x4(%rax) # 7 bytes
> je 1f # 2 bytes
> ud2 # 2 bytes
> 1: call __x86_indirect_thunk_rax # 5 bytes

Note that the compiler might not emit this *exact* sequence of
instructions. For example, Clang generates this for events_sysfs_show
with the modified KCFI patch:

2274: cmpl $0x4d7bed9e,-0x4(%r11)
227c: jne 22c0 <events_sysfs_show+0x6c>
227e: call 2283 <events_sysfs_show+0x2f>
227f: R_X86_64_PLT32 __x86_indirect_thunk_r11-0x4
...
22c0: ud2

In this case the function has two indirect calls and Clang seems to
prefer to emit just one ud2.

> .align 16
> .byte 0xef, 0xbe, 0xad, 0xde # 4 bytes
> func:
> endbr # 4 bytes

Here func is no longer aligned to 16 bytes, in case that's important.

> Further, Andrew put in the request for __attribute__((cfi_seed(blah)))
> to allow distinguishing indirect functions with otherwise identical
> signature; eg. cookie = hash32(blah##signature).

Sounds reasonable.

> Did I miss anything? Got anything wrong?

How would you like to deal with the 4-byte hashes in objtool? We
either need to annotate all function symbols in the kernel, or we need
a way to distinguish the hashes from random instructions, so we can
also have functions that don't have a type hash.

Sami