Re: [PATCH 7/9] x86/unwind/orc: Fall back to using frame pointers for generated code

[Josh Poimboeuf]

On Thu, Jun 13, 2019 at 11:00:09PM -0700, Alexei Starovoitov wrote:
> > +	if (src_reg == BPF_REG_FP) {
> > +		/*
> > +		 * If the value was copied from RBP (real frame pointer),
> > +		 * adjust it to the BPF program's frame pointer value.
> > +		 *
> > +		 * add dst, -40
> > +		 */
> > +		EMIT4(add_1mod(0x48, dst_reg), 0x83, add_1reg(0xC0, dst_reg),
> > +		      0xD8);
> > +	}
> > +
> 
> That won't work. Any register can point to a stack.

Right, but if the stack pointer comes from BPF_REG_FP then won't the
above correct it?  Then if the pointer gets passed around to other
registers it will have the correct value.  Or did I miss your point?

> The register can point to a stack of a different JITed function as well.

Do you mean tail calls?  Or something else?  For tail calls the stack is
shared and the stack layout is the same.

-- 
Josh