Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

[Josh Poimboeuf]

On Thu, Nov 29, 2018 at 10:42:10AM +0100, Peter Zijlstra wrote:
> On Wed, Nov 28, 2018 at 10:05:54PM -0800, Andy Lutomirski wrote:
> 
> > >> +static void static_call_bp_handler(struct pt_regs *regs, void *_data)
> > >> +{
> > >> +    struct static_call_bp_data *data = _data;
> > >> +
> > >> +    /*
> > >> +     * For inline static calls, push the return address on the stack so the
> > >> +     * "called" function will return to the location immediately after the
> > >> +     * call site.
> > >> +     *
> > >> +     * NOTE: This code will need to be revisited when kernel CET gets
> > >> +     *       implemented.
> > >> +     */
> > >> +    if (data->ret) {
> > >> +        regs->sp -= sizeof(long);
> > >> +        *(unsigned long *)regs->sp = data->ret;
> > >> +    }
> > 
> > You canât do this.  Depending on the alignment of the old RSP, which
> > is not guaranteed, this overwrites regs->cs.  IRET goes boom.
> 
> I don't get it; can you spell that out?

I don't quite follow that either.  Maybe Andy is referring to x86-32,
for which regs->sp isn't actually saved: see kernel_stack_pointer().

This code is 64-bit only so that's not a concern.

> The way I understand it is that we're at a location where a "E8 - Near
> CALL" instruction should be, and thus RSP should be the regular kernel
> stack, and the above simply does "PUSH ret", which is what that CALL
> would've done too.

Right.

-- 
Josh