Re: [PATCH 2/3] x86: add some better documentation for probe_kernel_address()

From: Andy Lutomirski
Date: Fri Jul 22 2016 - 14:23:13 EST

On Fri, Jul 22, 2016 at 11:18 AM, Dave Hansen <dave@xxxxxxxx> wrote:
> On 07/22/2016 11:10 AM, Andy Lutomirski wrote:
>> On Jul 22, 2016 11:03 AM, "Dave Hansen" <dave@xxxxxxxx> wrote:
>>> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
>>> probe_kernel_address() has an unfortunate name since it is used
>>> to probe kernel *and* userspace addresses. Add a comment
>>> explaining some of the situation to help the next developer who
>>> might make the silly assumption that it is for probing kernel
>>> addresses.
>> This can't work on architectures like s390 that have separate,
>> overlapping user and kernel address spaces. Maybe we should fix x86
>> to stop abusing it and use get_user instead. (In which case, your new
>> function should be called get_user_insn_byte or similar.)
> Urg.
> But can't the x86 use in no_context() be called from a kernel-initiated
> fault? Like a prefetch instruction to a vmalloc() page that we needed
> to do a vmalloc fault for?
> In either case, it would be awfully nice to have the clarity about
> exactly what is being probed. The other 2 calls to is_prefetch() do
> appear to be userspace-only.


Perhaps we should pass a bool is_kernel into is_prefetch().


Andy Lutomirski
AMA Capital Management, LLC