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

[Fangrui Song]

On Fri, Oct 21, 2022 at 10:39 AM Sami Tolvanen <samitolvanen at google.com> wrote:
>
> On Fri, Oct 21, 2022 at 8:56 AM Mark Rutland <mark.rutland at 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.

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.


-- 
宋方睿