Re: kCFI && patchable-function-entry=M,N

[Sami Tolvanen <samitolvanen@google.com>]

Hi folks,

On Mon, Oct 24, 2022 at 12:24:11PM +0100, Mark Rutland wrote:
> On Sat, Oct 22, 2022 at 04:57:18PM +0200, Peter Zijlstra wrote:
> > On Fri, Oct 21, 2022 at 04:56:20PM +0100, Mark Rutland wrote:
> > > Hi,
> > > 
> > > For arm64, I'd like to use -fatchable-function-entry=M,N (where N > 0), for our
> > > ftrace implementation, which instruments *some* but not all functions.
> > > Unfortuntately, this doesn't play nicely with -fsanitize=kcfi, as instrumented
> > > and non-instrumented functions don't agree on where the type hash should live
> > > relative to the function entry point, making them incompatible with one another.
> > > AFAICT, there's no mechanism today to get them to agree.
> > > 
> > > Today we use -fatchable-function-entry=2, which happens to avoid this.
> > 
> > > ... but I understand that for x86, folk want the pre-function NOPs to
> > > fall-through into the body of the function.
> > 
> > Yep.
> > 
> > > Is there any mechanism today that we could use to solve this, or could we
> > > extend clang to have some options to control this behaviour?
> > 
> > So the main pain-point for you is differentiating between function with
> > notrace and those without it, right?
> > 
> > That is; suppose you (like x86) globally do:
> > -fpatchable-function-entry=4,2 to get a consistent function signature,
> > you're up a creek because you use the __patchable_function_entries
> > section to drive ftrace and now every function will have it.
> > 
> > So perhaps something like:
> > 
> >  -fpatchable-function-entry=N,M,sectionname
> > 
> > would help, then you can have notrace be the same layout, except a
> > different section. Eg. something like:
> > 
> >  #define notrace __attribute__((patchable_function_entry(4,2,__notrace_function_entries)))
> 
> FWIW, I think that'd work for me, and that was roughly my original proposal on
> IRC. My only concern with this approach is code size, since all uninstrumented
> functions gain some point less prefix NOPs.

It took me a couple of years to find the time to look into this,
but here's a Clang patch I committed yesterday that adds support
for a section parameter:

https://github.com/llvm/llvm-project/commit/acc6bcdc504ad2e8c09a628dc18de0067f7344b8

Sami