linux-kernel - Re: [PATCH V3 5/9] objtool: Add return address unwind hints

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Date:   Tue, 14 Apr 2020 18:40:12 +0200
From:   Alexandre Chartre <alexandre.chartre@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
        jthierry@...hat.com, tglx@...utronix.de
Subject: Re: [PATCH V3 5/9] objtool: Add return address unwind hints

On 4/14/20 6:16 PM, Peter Zijlstra wrote:
> On Tue, Apr 14, 2020 at 12:36:14PM +0200, Alexandre Chartre wrote:
>> Add the UNWIND_HINT_RADDR_DELETE and UNWIND_HINT_RADDR_ALTER unwind
>> hint macros to flag instructions which remove or modify return
>> addresses.
> 
> I'm confused by this thing; why? AFAICT the rest of the patches are
> actually simpler without this one.
> 

This is required to indicate to objtool that assembly instructions are
changing return addresses. For example, in patch 8:

For retpoline:

@@ -88,6 +96,7 @@
  	lfence
  	jmp	.Lspec_trap_\@
  .Ldo_rop_\@:
+	UNWIND_HINT_RADDR_ALTER
  	mov	\reg, (%_ASM_SP)
  	ret
  .endm

The unwind hint indicates that the return address has been altered, so the
code won't return to the return address which was on the stack.

Also for RSB stuffing, this is used to indicated that the stack adjustment
instruction (add $(BITS_PER_LONG/8) * 2, sp) is actually removing the last
two return addresses.

alex.