Re: [PATCH v2] livepatch: account for patch offset when applying NOP patch

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.03.2022 10:01, Roger Pau Monné wrote:
> On Thu, Mar 31, 2022 at 08:42:47AM +0200, Jan Beulich wrote:
>> On 30.03.2022 19:04, Roger Pau Monné wrote:
>>> On Wed, Mar 30, 2022 at 01:05:31PM +0200,>> --- a/xen/arch/x86/livepatch.c
>>>> +++ b/xen/arch/x86/livepatch.c
>>>> @@ -157,9 +157,15 @@ void noinline arch_livepatch_apply(struc
>>>>       * loaded hotpatch (to avoid racing against other fixups 
>>>> adding/removing
>>>>       * ENDBR64 or similar instructions).
>>>>       */
>>>> -    if ( is_endbr64(old_ptr) || is_endbr64_poison(func->old_addr) )
>>>> +    if ( len >= ENDBR64_LEN &&
>>>
>>> Sorry, didn't realize before, but shouldn't this check be using
>>> old_size instead of len (which is based on new_size)?
>>
>> Yes and no: In principle yes, but with len == func->new_size in the NOP
>> case, and with arch_livepatch_verify_func() guaranteeing new_size <=
>> old_size, the check is still fine for that case. Plus: If new_size was
>> less than 4 _but_ there's an ENDBR64 at the start, what would we do? I
>> think there's more that needs fixing in this regard. So I guess I'll
>> make a v3 with this extra fix dropped and with the livepatch_insn_len()
>> invocation simply moved. After all the primary goal is to get the
>> stable trees unstuck.
> 
> Right, I agree to try and get the stable trees unblocked ASAP.
> 
> I think the check for ENDBR is only relevant when we are patching the
> function with a jump, otherwise the new replacement code should
> contain the ENDBR instruction already?

No, the "otherwise" case is when we're NOP-ing out code, i.e. when
there's no replacement code at all. In this case we need to leave
intact the ENDBR, and new_size being less than 4 is bogus afaict in
case there actually is an ENDBR.

Jan