On Mon, Sep 14, 2020 at 02:07:19PM -0700, Sean Christopherson wrote:
> 16-byte aligned to correctly mimic CPU behavior when vectoring an IRQ/NMI.
> When not changing stack, the CPU aligns RSP before pushing the frame.
16 byte alignment is not needed for the internal kernel ABI because it doesn't use
SSE.
-Andi
>
> The above shenanigans work because the x86-64 ABI also requires RSP to be
> 16-byte aligned prior to CALL. RSP is thus 8-byte aligned due to CALL
> pushing the return IP, and so creating the stack frame by pushing RBP makes
> it 16-byte aliagned again.
>
> > > +
> > > +#ifdef CONFIG_X86_64
> > > + push $__KERNEL_DS
> > > + push %_ASM_BP
> > > +#endif
> > > + pushf
> > > + push $__KERNEL_CS
> > > + CALL_NOSPEC _ASM_ARG1
> > > +
> > > + /*
> > > + * "Restore" RSP from RBP, even though IRET has already unwound RSP to
> > > + * the correct value. objtool doesn't know the target will IRET and so
> > > + * thinks the stack is getting walloped (without the explicit restore).
> > > + */
> > > + mov %_ASM_BP, %rsp
> > > + pop %_ASM_BP
> > > + ret
> >
> > BTW, there *is* actually an unwind hint for this situation:
> > UNWIND_HINT_RET_OFFSET.
>
> I played with that one, but for the life of me couldn't figure out how to
> satisfy both the "stack size" and "cfa.offset" checks. In the code below,
> cfa.offset will be 8, stack_size will be 40 and initial_func_cfi.cfa.offset
> will be 8. But rereading this, I assume I missed something that would allow
> maniuplating cfa.offset? Or maybe I botched my debugging?
>
> static bool has_modified_stack_frame(struct instruction *insn, struct insn_state *state)
> {
> ...
>
> if (cfi->cfa.offset != initial_func_cfi.cfa.offset + ret_offset)
> return true;
>
> if (cfi->stack_size != initial_func_cfi.cfa.offset + ret_offset)
> return true;
>
> ...
> }
>
> > So you might be able to do something like the following (depending on
> > what your alignment requirements actually are):
> >
> > SYM_FUNC_START(vmx_do_interrupt_nmi_irqoff)
> > #ifdef CONFIG_X86_64
> > push $__KERNEL_DS
> > push %_ASM_BP
> > #endif
> > pushf
> > push $__KERNEL_CS
> > CALL_NOSPEC _ASM_ARG1
> >
> > /* The call popped the pushes */
> > UNWIND_HINT_RET_OFFSET sp_offset=32
> >
> > ret
> > SYM_FUNC_END(vmx_do_interrupt_nmi_irqoff)
> >
> > --
> > Josh
> >