Re: [PATCH] x86/mm: fix detection of last L1 entry in modify_xen_mappings_lite()

[Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>]

On 11/03/2024 11:01, Andrew Cooper wrote:
> On 11/03/2024 10:54 am, Roger Pau Monne wrote:
>> The current logic to detect when to switch to the next L1 table is 
>> incorrectly
>> using l2_table_offset() in order to notice when the last entry on the current
>> L1 table has been reached.
>>
>> It should instead use l1_table_offset() to check whether the index has 
>> wrapped
>> to point to the first entry, and so the next L1 table should be used.
>>
>> Fixes: 8676092a0f16 ('x86/livepatch: Fix livepatch application when CET is 
>> active')
>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> 
>> ---
>> This fixes the osstest livepatch related crash, we have been lucky so far 
>> that
>> the .text section didn't seem to have hit this.
> 
> I'm very surprised too.
> 

The two stages for software debugging:
  1. I really don't see how it can possibly fail.
  2. I really don't see how it could have possibly worked.