RE: [PATCH v5 00/15] x86: Add support for Clang CFI

[David Laight]

From: Mark Rutland
> Sent: 27 October 2021 14:18
> 
> On Wed, Oct 27, 2021 at 12:55:17PM +0000, David Laight wrote:
> > From: Mark Rutland
> > > Sent: 27 October 2021 13:05
> > ...
> > > Taking a step back, it'd be nicer if we didn't have the jump-table shim
> > > at all, and had some SW landing pad (e.g. a NOP with some magic bytes)
> > > in the callees that the caller could check for. Then function pointers
> > > would remain callable in call cases, and we could explcitly add landing
> > > pads to asm to protect those. I *think* that's what the grsecurity folk
> > > do, but I could be mistaken.
> >
> > It doesn't need to be a 'landing pad'.
> > The 'magic value' could be at 'label - 8'.
> 
> Sure; I'd intended to mean the general case of something at some fixed
> offset from the entrypoint, either before or after, potentially but not
> necessarily inline in the executed instruction stream.

What you really want is to be able to read the value using the I-cache
so as not to pollute the D-cache with code bytes and to avoid having
both an I-cache and D-cache miss at the same time for the same memory.

Even if the I-cache read took an extra clock (or two) I suspect it
would be an overall gain.
This is also true for code that uses pc-relative instructions to
read constants - common in arm-64.

Not sure any hardware lets you do that though :-(

	David

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