Re: mmotm 2020-05-05-15-28 uploaded (objtool warning)

[Randy Dunlap]

On 5/8/20 3:38 AM, Peter Zijlstra wrote:
> On Tue, May 05, 2020 at 10:40:43PM -0700, Randy Dunlap wrote:
>> On 5/5/20 3:29 PM, akpm@xxxxxxxxxxxxxxxxxxxx wrote:
>>> The mm-of-the-moment snapshot 2020-05-05-15-28 has been uploaded to
>>>
>>>    http://www.ozlabs.org/~akpm/mmotm/
>>>
>>> mmotm-readme.txt says
>>>
>>> README for mm-of-the-moment:
>>>
>>> http://www.ozlabs.org/~akpm/mmotm/
>>>
>>> This is a snapshot of my -mm patch queue.  Uploaded at random hopefully
>>> more than once a week.
>>>
>>
>>
>> on x86_64:
>>
>> arch/x86/hyperv/hv_apic.o: warning: objtool: hv_apic_write()+0x25: alternative modifies stack
> 
> Wheee... this seems to have cured it for me.
> 
> ---
> Subject: objtool: Allow no-op CFI ops in alternatives
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Fri May 8 12:34:33 CEST 2020
> 
> Randy reported a false-positive: "alternative modifies stack".
> 
> What happens is that:
> 
> 	alternative_io("movl %0, %P1", "xchgl %0, %P1", X86_BUG_11AP,
>  13d:   89 9d 00 d0 7f ff       mov    %ebx,-0x803000(%rbp)
> 
> decodes to an instruction with CFI-ops because it modifies RBP.
> However, due to this being a !frame-pointer build, that should not in
> fact change the CFI state.
> 
> So instead of dis-allowing any CFI-op, verify the op would've actually
> changed the CFI state.
> 
> Fixes: 7117f16bf460 ("objtool: Fix ORC vs alternatives")
> Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Acked-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Tested-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>

Thanks.

> ---
>  tools/objtool/check.c |   11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -2078,17 +2078,18 @@ static int handle_insn_ops(struct instru
>  	struct stack_op *op;
>  
>  	list_for_each_entry(op, &insn->stack_ops, list) {
> +		struct cfi_state old_cfi = state->cfi;
>  		int res;
>  
> -		if (insn->alt_group) {
> -			WARN_FUNC("alternative modifies stack", insn->sec, insn->offset);
> -			return -1;
> -		}
> -
>  		res = update_cfi_state(insn, &state->cfi, op);
>  		if (res)
>  			return res;
>  
> +		if (insn->alt_group && memcmp(&state->cfi, &old_cfi, sizeof(struct cfi_state))) {
> +			WARN_FUNC("alternative modifies stack", insn->sec, insn->offset);
> +			return -1;
> +		}
> +
>  		if (op->dest.type == OP_DEST_PUSHF) {
>  			if (!state->uaccess_stack) {
>  				state->uaccess_stack = 1;
> 


-- 
~Randy