Re: [PATCH] cfi: rust: pass -Zpatchable-function-entry on all architectures

Alice Ryhl
2024-10-10T12:29:32Z

On Thu, Oct 10, 2024 at 12:45 PM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Wed, Oct 09, 2024 at 10:15:35PM +0200, Alice Ryhl wrote:
> > On Wed, Oct 9, 2024 at 7:43 PM Mark Rutland <mark.rutland@arm.com> wrote:
> > >
> > > Hi Alice,
> > >
> > > On Tue, Oct 08, 2024 at 05:37:16PM +0000, Alice Ryhl wrote:
> > > > The KCFI sanitizer stores the CFI tag of a function just before its
> > > > machine code. However, the patchable-function-entry flag can be used to
> > > > introduce additional nop instructions before the machine code, taking up
> > > > the space that normally holds the CFI tag.
> > >
> > > To clarify, when you say "before the machine code", do you mean when
> > > NOPs are placed before the function entry point? e.g. if we compiled
> > > with -fpatchable-function-entry=M,N where N > 0? I'll refer tho this as
> > > "pre-function NOPs" below.
> > >
> > > There's an existing incompatibility between CFI and pre-function NOPs
> > > for C code, because we override -fpatchable-function-entry on a
> > > per-function basis (e.g. for noinstr and notrace), and we don't
> > > currently have a mechanism to ensure the CFI tag is in the same place
> > > regardless. This is why arm64 has CONFIG_DYNAMIC_FTRACE_WITH_CALL_OPS
> > > depend on !CFI.
> > >
> > > For C code at least, just using regular -fpatchable-function-entry=M or
> > > -fpatchable-function-entry=M,0 shouldn't change the location of the CFI
> > > tag relative to the function entrypoint, and so should have no adverse
> > > effect on CFI.
> > >
> > > Is Rust any different here?
> >
> > Ah, no it shouldn't be. Sami can you confirm?
> >
> > > > In this case, a backwards offset is applied to the CFI tag to move
> > > > them out of the way of the nop instructions. To ensure that C and Rust
> > > > agree on the offset used by CFI tags, pass the
> > > > -Zpatchable-function-entry to rustc whenever it is passed to the C
> > > > compiler.
> > >
> > > As above, I suspect this isn't necessary to make CFI work, for any case
> > > that works with C today, due to -fpatchable-funtion-entry being
> > > overridden on a per-function basis. Are you seeing a problem in
> > > practice, or was this found by inspection?
> > >
> > > However IIUC this will allow rust to be traced via ftrace (assuming rust
> > > records the instrumented locations as gcc and clang do); is that the
> > > case? Assuming so, is there any ABI difference that might bite us? On
> > > arm64 we require that anything marked instrumented with
> > > patchable-function-entry strictly follows the AAPCS64 calling convention
> > > and our ftrace trampolines save/restore the minimal set of necessary
> > > registers, and I don't know how rust whether rust will behave the same
> > > or e.g. use specialized calling conventions internally.
> >
> > Well, I was told that it's a problem and was able to trigger a failure
> > on x86. I didn't manage to trigger one on arm64, but I wasn't sure
> > whether that was me doing something wrong, or whether the problem only
> > exists on x86. We already have the flag on x86 for FINEIBT,
>
> I believe that hte problem only exists on x86, becaause they use
> patchable-function-entry for their FINEIBT patching (and use -mfentry
> for ftrace), whereas everyone else uses patchable-function-entry for
> ftrace.
>
> > but I thought on the off chance that it's not a problem in practice on
> > arm, it still doesn't hurt to add the flag.
>
> It won't adversely affect CFI, but it will open up rust code for ftrace,
> so I'm not sure that "it doesn't hurt".
>
> AFAICT at the moment this isn't necessary for CFI, so can we drop this
> patch for now?

I'm okay with dropping this patch for now.

Alice