linux-kernel - Re: [PATCH 2/3] x86: add some better documentation for probe_kernel

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Message-ID: <57926365.5040307@sr71.net>
Date:	Fri, 22 Jul 2016 11:18:13 -0700
From:	Dave Hansen <dave@...1.net>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 2/3] x86: add some better documentation for
 probe_kernel_address()

On 07/22/2016 11:10 AM, Andy Lutomirski wrote:
> On Jul 22, 2016 11:03 AM, "Dave Hansen" <dave@...1.net> wrote:
>> From: Dave Hansen <dave.hansen@...ux.intel.com>
>>
>> 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.