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

[Menglong Dong <menglong8.dong@gmail.com>]

On Tue, Mar 4, 2025 at 3:47 PM Menglong Dong <menglong8.dong@gmail.com> wrote:
>
> On Tue, Mar 4, 2025 at 2:16 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Tue, Mar 04, 2025 at 06:38:53AM +0100, Peter Zijlstra wrote:
> > > On Tue, Mar 04, 2025 at 09:10:12AM +0800, Menglong Dong wrote:
> > > > 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:
> > >
> > > *blink*, wtf is clang smoking.
> > >
> > > I mean, you're right, this is what it is doing, but that is somewhat
> > > unexpected. Let me go look at clang source, this is insane.
> >
> > Bah, this is because assemblers are stupid :/
> >
> > There is no way to tell them to have foo aligned such that there are at
> > least N bytes free before it.
> >
> > So what kCFI ends up having to do is align the __cfi symbol to the
> > function alignment, and then stuff enough nops in to make the real
> > symbol meet alignment.
> >
> > And the end result is utter insanity.
> >
> > I mean, look at this:
> >
> >       50:       2e e9 00 00 00 00       cs jmp 56 <__traceiter_initcall_level+0x46>     52: R_X86_64_PLT32      __x86_return_thunk-0x4
> >       56:       66 2e 0f 1f 84 00 00 00 00 00   cs nopw 0x0(%rax,%rax,1)
> >
> > 0000000000000060 <__cfi___probestub_initcall_level>:
> >       60:       90                      nop
> >       61:       90                      nop
> >       62:       90                      nop
> >       63:       90                      nop
> >       64:       90                      nop
> >       65:       90                      nop
> >       66:       90                      nop
> >       67:       90                      nop
> >       68:       90                      nop
> >       69:       90                      nop
> >       6a:       90                      nop
> >       6b:       b8 b1 fd 66 f9          mov    $0xf966fdb1,%eax
> >
> > 0000000000000070 <__probestub_initcall_level>:
> >       70:       2e e9 00 00 00 00       cs jmp 76 <__probestub_initcall_level+0x6>      72: R_X86_64_PLT32      __x86_return_thunk-0x4
> >
> >
> > That's 21 bytes wasted, for no reason other than that asm doesn't have a
> > directive to say: get me a place that is M before N alignment.
> >
> > Because ideally the whole above thing would look like:
> >
> >       50:       2e e9 00 00 00 00       cs jmp 56 <__traceiter_initcall_level+0x46>     52: R_X86_64_PLT32      __x86_return_thunk-0x4
> >       56:       66 2e 0f 1f 84          cs nopw (%rax,%rax,1)
> >
> > 000000000000005b <__cfi___probestub_initcall_level>:
> >       5b:       b8 b1 fd 66 f9          mov    $0xf966fdb1,%eax
> >
> > 0000000000000060 <__probestub_initcall_level>:
> >       60:       2e e9 00 00 00 00       cs jmp 76 <__probestub_initcall_level+0x6>      72: R_X86_64_PLT32      __x86_return_thunk-0x4
>
> Hi, peter. Thank you for the testing, which is quite helpful
> to understand the whole thing.
>
> I was surprised at this too. Without CALL_PADDING, the cfi is
> nop(11) + mov; with CALL_PADDING, the cfi is mov + nop(11),
> which is weird, as it seems that we can select CALL_PADDING if
> CFI_CLANG to make things consistent. And I  thought that it is
> designed to be this for some reasons :/
>
> Hmm......so what should we do now? Accept and bear it,
> or do something different?
>
> I'm good at clang, so the solution that I can think of is how to

*not good at*

> bear it :/
>
> According to my testing, the text size will increase:
>
> ~2.2% if we make FUNCTION_PADDING_BYTES 27 and select
> FUNCTION_ALIGNMENT_16B.
>
> ~3.5% if we make FUNCTION_PADDING_BYTES 27 and select
> FUNCTION_ALIGNMENT_32B.
>
> We don't have to select FUNCTION_ALIGNMENT_32B, so the
> worst case is to increase ~2.2%.
>
> What do you think?
>
> Thanks!
> Menglong Dong
>
> >
> >
> >