linux-kernel - Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls in _

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Message-ID: <fp5amaygv37wxr6bglagljr325rsagllbabb62ow44kl3mznb6@gzk6nuukjgwv>
Date: Mon, 2 Jun 2025 22:43:33 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Sean Christopherson <seanjc@...gle.com>, 
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org, kys@...rosoft.com, haiyangz@...rosoft.com, 
	wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, pbonzini@...hat.com, 
	ardb@...nel.org, kees@...nel.org, Arnd Bergmann <arnd@...db.de>, 
	gregkh@...uxfoundation.org, linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, linux-efi@...r.kernel.org, samitolvanen@...gle.com, 
	ojeda@...nel.org, xin@...or.com
Subject: Re: [PATCH v2 00/13] objtool: Detect and warn about indirect calls
 in __nocfi functions

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