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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 31.03.2022 10:27, Roger Pau Monné wrote:
> On Thu, Mar 31, 2022 at 10:13:06AM +0200, Jan Beulich wrote:
>> 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.
> 
> Hm, so we never do in-place replacement of code, and we either
> introduce a jump to the new code or otherwise the function is not to
> be called anymore and hence we fill it with no-ops?

If it wasn't to be called anymore, it would be better to fill the
space with INT3, not NOP. I think the purpose isn't really to nop
out entire functions; it's just that the NOP testcase in the tree
does so.

> Shouldn't in the no-op filling case the call to add_nops be bounded by
> old_size and salso the memcpy to old_addr?
> 
> I'm not sure I understand why we use new_size when memcpy'ing into
> old_addr, or when filling the insn buffer.

I was wondering too - it would have seemed more natural to either
require old_size == new_size in this case, or to demand new_size == 0
matching new_addr == NULL. I'm afraid I have to rely on the livepatch
maintainers to answer your questions.

Jan