Re: [PATCH 1/2] x86/eventinj: Use global asm label for nested NMI IP address verification

[Chao Gao <chao.gao@intel.com>]

On Tue, Sep 16, 2025 at 12:10:46PM +0200, Mathias Krause wrote:
>Am 15.09.25 um 16:49 schrieb Chao Gao:
>> Use a global asm label to get the expected IP address for nested NMI
>> interception instead of reading a hardcoded offset from the stack.
>> 
>> the NMI test in eventinj.c verifies that a nested NMI occurs immediately at
>> the return address (IP register) in the IRET frame, as IRET opens the
>> NMI window. Currently, nested_nmi_iret_isr() reads the return address
>> using a magic offset (iret_stack[-3]), which is unclear and may break if
>> more values are pushed to the "iret_stack".
>> 
>> To improve readability, add a global 'ip_after_iret' label for the expected
>> return address, push it to the IRET frame, and verify it matches the
>> interrupted address in the nested NMI handler.
>> 
>> Signed-off-by: Chao Gao <chao.gao@intel.com>
>> ---
>>  x86/eventinj.c | 11 +++++++----
>>  1 file changed, 7 insertions(+), 4 deletions(-)
>> 
>> diff --git a/x86/eventinj.c b/x86/eventinj.c
>> index 6fbb2d0f..ec8a5ef1 100644
>> --- a/x86/eventinj.c
>> +++ b/x86/eventinj.c
>> @@ -127,12 +127,13 @@ static void nmi_isr(struct ex_regs *r)
>>  }
>>  
>>  unsigned long *iret_stack;
>> +extern char ip_after_iret[];
>>  
>>  static void nested_nmi_iret_isr(struct ex_regs *r)
>>  {
>>  	printf("Nested NMI isr running rip=%lx\n", r->rip);
>>  
>> -	if (r->rip == iret_stack[-3])
>> +	if (r->rip == (unsigned long)ip_after_iret)
>
>This change basically eliminates the need for the global
>'ip_after_iret', it can be local to nmi_iret_isr() now.

You meant 'iret_stack', right? if so, sure, I will make it local to
nmi_iret_isr().