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

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Oct 21, 2022 at 09:14:41PM -0700, Fangrui Song wrote:
> On Fri, Oct 21, 2022 at 10:39 AM Sami Tolvanen <samitolvanen@google.com> wrote:
> >
> > On Fri, Oct 21, 2022 at 8:56 AM Mark Rutland <mark.rutland@arm.com> 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.
> >
> > Yes, the current implementation assumes that if prefix nops are used,
> > all functions have the same number of them.
> >
> > > 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?
> >
> > I don't think there's a mechanism to work around the issue right now,
> > but we could just change where the hash is emitted on arm64.
> >
> > > It would also be helpful to have a symbol before both the hash and pre-function
> > > NOPs so that we can filter those out of probes patching (I see that x86 does
> > > this with the __cfi_function symbol).
> >
> > Adding a symbol before the hash isn't a problem, but if we move the
> > hash and want the symbol to be placed before the prefix nops as well,
> > we might need a flag to control this. Fangrui, what do you think?
> >
> > Sami
> 
> Since the kcfi code expects the hash to appear in a specific location
> so that an instrumented indirect jump can reliably obtain the hash.
> For a translation unit `-fpatchable-function-entry=N,M` may be
> specified or not, and we want both to work. Therefore, I agree that a
> consistent hash location will help. This argument favors placing M
> nops before the hash. The downside is a restriction on how the M nops
> can be used. Previously if M>0, the runtime code needs to check
> whether a BTI exists to locate the N-M after-function-entry NOPs. If
> the hash appears after the M nops, the runtime code needs to
> additionally knows whether the hash exists. My question is how to
> reliably detect this.

That's a fair point.

For detecting BTI we can scan the binary for BTI/NOP at M instructions into the
patch-site, but a similar approach won't be reliable for the type hash since
the type hash itself could have the same bit pattern as an instruction.

> If there is motivation using M>0, I'd like to know the concrete code
> sequence for `-fpatchable-function-entry=N,M` and how the runtime code
> detects the NOPs with optional hash and optional BTI.

For the BTI case alone, I have code at:

  https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/commit/?h=arm64/ftrace/per-callsite-ops&id=272a580fd5b7acc31747505d71530cee7cc2837d

... the gist being that it checks the instruction M insns after the start of
the patch site.

For the type hash, I think there are a few options, e.g.

* Restricting the type hash to a set of values that can be identified (e.g.
  encoding those as a permanently-undefined UDF with a 16-bit immediate).

* Adding options to record additional metadata along with the pointer to the
  patch-site in the __patchable_function_entries section.

* Adding an option to record patch-site variants to sub-sections of the
  __patchable_function_entries section, so that at link time these can be
  grouped separately, e.g.

  * __patchable_function_entries.???		// no BTI, no type hash
  * __patchable_function_entries.bti		// has BTI
  * __patchable_function_entries.bti_cfi	// has BTI and type hash
  * __patchable_function_entries.cfi		// has type hash

Do any of those approaches sound plausible to you?

Thanks,
Mark.