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

[Andy Lutomirski]

On Thu, Nov 29, 2018 at 12:24 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> > Alternatively, we could actually emulate call instructions like this:
> >
> > void __noreturn jump_to_kernel_pt_regs(struct pt_regs *regs, ...)
> > {
> >   struct pt_regs ptregs_copy = *regs;
> >   barrier();
> >   *(unsigned long *)(regs->sp - 8) = whatever;  /* may clobber old
> > regs, but so what? */
> >   asm volatile ("jmp return_to_alternate_ptregs");
> > }
> >
> > where return_to_alternate_ptregs points rsp to the ptregs and goes
> > through the normal return path.  It's ugly, but we could have a test
> > case for it, and it should work fine.
>
> Is that really any better than my patch to create a gap in the stack
> (modified for kernel space #BP only)?
>

I tend to prefer a nice local hack like mine over a hack that further
complicates the entry in general.  This is not to say I'm thrilled by
my idea either.