Re: [PATCH v4 1/4] x86/ibt: factor out cfi and fineibt offset

Menglong Dong
2025-03-04T01:11:50Z

On Tue, Mar 4, 2025 at 12:55 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Mar 03, 2025 at 09:28:34PM +0800, Menglong Dong wrote:
> > For now, the layout of cfi and fineibt is hard coded, and the padding is
> > fixed on 16 bytes.
> >
> > Factor out FINEIBT_INSN_OFFSET and CFI_INSN_OFFSET. CFI_INSN_OFFSET is
> > the offset of cfi, which is the same as FUNCTION_ALIGNMENT when
> > CALL_PADDING is enabled. And FINEIBT_INSN_OFFSET is the offset where we
> > put the fineibt preamble on, which is 16 for now.
> >
> > When the FUNCTION_ALIGNMENT is bigger than 16, we place the fineibt
> > preamble on the last 16 bytes of the padding for better performance, which
> > means the fineibt preamble don't use the space that cfi uses.
> >
> > The FINEIBT_INSN_OFFSET is not used in fineibt_caller_start and
> > fineibt_paranoid_start, as it is always "0x10". Note that we need to
> > update the offset in fineibt_caller_start and fineibt_paranoid_start if
> > FINEIBT_INSN_OFFSET changes.
> >
> > Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
>
> I'm confused as to what exactly you mean.
>
> Preamble will have __cfi symbol and some number of NOPs right before
> actual symbol like:
>
> __cfi_foo:
> mov $0x12345678, %reg
> nop
> nop
> nop
> ...
> foo:
>
> FineIBT must be at foo-16, has nothing to do with performance. This 16
> can also be spelled: fineibt_preamble_size.
>
> The total size of the preamble is FUNCTION_PADDING_BYTES + CFI_CLANG*5.
>
> If you increase FUNCTION_PADDING_BYTES by another 5, which is what you
> want I think, then we'll have total preamble of 21 bytes; 5 bytes kCFI,
> 16 bytes nop.

Hello, sorry that I forgot to add something to the changelog. In fact,
I don't add extra 5-bytes anymore, which you can see in the 3rd patch.

The thing is that we can't add extra 5-bytes if CFI is enabled. Without
CFI, we can make the padding space any value, such as 5-bytes, and
the layout will be like this:

__align:
nop
nop
nop
nop
nop
foo: -- __align +5

However, the CFI will always make the cfi insn 16-bytes aligned. When
we set the FUNCTION_PADDING_BYTES to (11 + 5), the layout will be
like this:

__cfi_foo:
nop (11)
mov $0x12345678, %reg
nop (16)
foo:

and the padding space is 32-bytes actually. So, we can just select
FUNCTION_ALIGNMENT_32B instead, which makes the padding
space 32-bytes too, and have the following layout:

__cfi_foo:
mov $0x12345678, %reg
nop (27)
foo:

And the layout will be like this if fineibt and function metadata is both
used:

__cfi_foo:
mov -- 5 -- cfi, not used anymore if fineibt is used
nop
nop
nop
mov -- 5 -- function metadata
nop
nop
nop
fineibt -- 16 -- fineibt
foo:
nopw -- 4
......

The things that I make in this commit is to make sure that
the code in arch/x86/kernel/alternative.c can find the location
of cfi hash and fineibt depends on the FUNCTION_ALIGNMENT.
the offset of cfi and fineibt is different now, so we need to do
some adjustment here.

In the beginning, I thought to make the layout like this to ensure
that the offset of cfi and fineibt the same:

__cfi_foo:
fineibt -- 16 -- fineibt
mov -- 5 -- function metadata
nop(11)
foo:
nopw -- 4
......

The adjustment will be easier in this mode. However, it may have
impact on the performance. That way I say it doesn't impact the
performance in this commit.

Sorry that I didn't describe it clearly in the commit log, and I'll
add the things above to the commit log too in the next version.

Thanks!
Menglong Dong

>
> Then kCFI expects hash to be at -20, while FineIBT must be at -16.
>
> This then means there is no unambiguous hole for you to stick your
> meta-data thing (whatever that is).
>
> There are two options: make meta data location depend on cfi_mode, or
> have __apply_fineibt() rewrite kCFI to also be at -16, so that you can
> have -21 for your 5 bytes.
>
> I think I prefer latter.
>
> In any case, I don't think we need *_INSN_OFFSET. At most we need
> PREAMBLE_SIZE.
>
> Hmm?