Re: WARNING in memtype_reserve

[Thomas Gleixner]

Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> writes:
> On Sat, 9 May 2020, Thomas Gleixner wrote:
>
>> Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
>> > On Sat, May 09, 2020 at 12:20:14AM -0700, syzbot wrote:
>> >> memtype_reserve failed: [mem 0xffffffffff000-0x00008fff], req write-back
>> >> WARNING: CPU: 1 PID: 7025 at arch/x86/mm/pat/memtype.c:589 memtype_reserve+0x69f/0x820 arch/x86/mm/pat/memtype.c:589
>> >
>> > So should memtype_reserve() not do a WARN if given invalid parameters as
>> > it can be triggered by userspace requests?
>> >
>> > A normal "invalid request" debug line is probably all that is needed,
>> > right?
>> 
>> I disagree. The callsite espcially if user space triggerable should not
>> attempt to ask for a reservation where start > end:
>> 
>>   >> memtype_reserve failed: [mem 0xffffffffff000-0x00008fff], req write-back
>> 
>> The real question is which part of the call chain is responsible for
>> this. That needs to be fixed.
>
> What about all the other ways in which a reservation request could be
> invalid?  The MM core already checks for these; what point is there in
> duplicating these checks in many places higher up the call chain?

Defensive programming rule #1: Check crap early but have the check which
ultimatively catches it at the last possible place as well.