Re: [RFC PATCH 00/17] objtool: add base support for arm64

[Ard Biesheuvel <ardb@kernel.org>]

On Fri, 22 Jan 2021 at 22:15, Madhavan T. Venkataraman
<madvenka@linux.microsoft.com> wrote:
>
>
>
> On 1/22/21 11:43 AM, Mark Brown wrote:
> > On Thu, Jan 21, 2021 at 12:54:52PM -0600, Josh Poimboeuf wrote:
> >
> >> 2) The shadow stack idea sounds promising -- how hard would it be to
> >>    make a prototype reliable unwinder?
> >
> > In theory it doesn't look too hard and I can't see a particular reason
> > not to try doing this - there's going to be edge cases but hopefully for
> > reliable stack trace they're all in areas where we would be happy to
> > just decide the stack isn't reliable anyway, things like nesting which
> > allocates separate shadow stacks for each nested level for example.
> > I'll take a look.
> >
>
> I am a new comer to this discussion and I am learning. Just have some
> questions. Pardon me if they are obvious or if they have already been
> asked and answered.
>
> Doesn't Clang already have support for a shadow stack implementation for ARM64?
> We could take a look at how Clang does it.
>
> Will there not be a significant performance hit? May be, some of it can be
> mitigated by using a parallel shadow stack rather than a compact one.
>
> Are there any longjmp style situations in the kernel where the stack is
> unwound by several frames? In these cases, the shadow stack must be unwound
> accordingly.
>

Hello Madhavan,

Let's discuss the details of shadow call stacks on a separate thread,
instead of further hijacking Julien's series.