Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in __nocfi functions

[Josh Poimboeuf]

On Thu, May 29, 2025 at 11:30:17AM +0200, Peter Zijlstra wrote:
> > > So the sequence of fail is:
> > > 
> > > 	push %rbp
> > > 	mov %rsp, %rbp	# cfa.base = BP
> > > 
> > > 	SAVE
> 
> 	sub    $0x40,%rsp
> 	and    $0xffffffffffffffc0,%rsp
> 
> This hits the 'older GCC, drap with frame pointer' case in OP_SRC_AND.
> Which means we then hard rely on the frame pointer to get things right.
> 
> However, per all the PUSH/POP_REGS nonsense, BP can get clobbered.
> Specifically the code between the CALL and POP %rbp below are up in the
> air. I don't think it can currently unwind properly there.

RBP is callee saved, so there's no need to pop it or any of the other
callee-saved regs.  If they were to change, that would break C ABI
pretty badly.  Maybe add a skip_callee=1 arg to POP_REGS?

-- 
Josh