Re: [PATCH v2 01/24] x86/xen: Mark cpu_bringup_and_idle() as dead_end_function

[Josh Poimboeuf]

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@xxxxxxxxxxxxx>

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