linux-kernel - Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end

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Message-ID: <20210820192224.ytrr6ybuuwegbeov@treble>
Date:   Fri, 20 Aug 2021 12:22:24 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, joro@...tes.org,
        boris.ostrovsky@...cle.com, jgross@...e.com, x86@...nel.org,
        mbenes@...e.com, rostedt@...dmis.org, dvyukov@...gle.com,
        elver@...gle.com
Subject: Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as
 dead_end_function

On Thu, Jun 24, 2021 at 11:41:00AM +0200, Peter Zijlstra wrote:
> The asm_cpu_bringup_and_idle() function is required to push the return
> value on the stack in order to make ORC happy, but the only reason
> objtool doesn't complain is because of a happy accident.
> 
> The thing is that asm_cpu_bringup_and_idle() doesn't return, so
> validate_branch() never terminates and falls through to the next
> function, which in the normal case is the hypercall_page. And that, as
> it happens, is 4095 NOPs and a RET.
> 
> Make asm_cpu_bringup_and_idle() terminate on it's own, by making the
> function it calls as a dead-end. This way we no longer rely on what
> code happens to come after.
> 
> Fixes: c3881eb58d56 ("x86/xen: Make the secondary CPU idle tasks reliable")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Looks right.  Only problem is, with my assembler I get this:

  arch/x86/kernel/head_64.o: warning: objtool: .text+0x5: unreachable instruction

Because gas insists on jumping over the page of nops...

0000000000000000 <asm_cpu_bringup_and_idle>:
       0:	e8 00 00 00 00       	callq  5 <asm_cpu_bringup_and_idle+0x5>
			1: R_X86_64_PLT32	cpu_bringup_and_idle-0x4
       5:	e9 f6 0f 00 00       	jmpq   1000 <xen_hypercall_set_trap_table>
       a:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
      11:	00 00 00 00 
      15:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
      1c:	00 00 00 00 
      20:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
      27:	00 00 00 00 
      2b:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
      32:	00 00 00 00 
      36:	66 66 2e 0f 1f 84 00 	data16 nopw %cs:0x0(%rax,%rax,1)
      3d:	00 00 00 00 

-- 
Josh