linux-kernel - Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/ data=journal

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Message-ID: <20151006092757.GA27419@gmail.com>
Date:	Tue, 6 Oct 2015 11:27:57 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Theodore Ts'o <tytso@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [REGRESSION] 998ef75ddb and aio-dio-invalidate-failure w/
 data=journal


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Oct 6, 2015 at 8:56 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > Yes, but note that those interfaces are x86 only at the moment, so they'd have 
> > to be factored out and generalized before we can use it in generic code.
> 
> ARM64 these days (as a part of ARM8.1) has "Privileged Access Never", which is 
> their name for SMAP. They do it somewhat similarly with an instruction to 
> clear/set the PAN bit in the pstate register.
> 
> So we really should strive to make this support generic, because by now it's not 
> x86-specific, and x86 and ARM64 together aren't exactly some odd special case..

Absolutely.

> I do in fact wonder if we should aim (eventually) for the rule that the "__" 
> versions of the user access functions should not do the SMAP/PAN thing, since 
> they have to be explicitly checked for pointer being valid anyway. And just make 
> the rule be that since you have to check for the pointer being valid, you might 
> as well also have to do the SMAP/PAN thing too.
> 
> We really should try get rid of _all_ uses of the "__" versions unless they are 
> very locally and obviously checked with access_ok(). We've had way too many 
> cases where people thought they were clever, and weren't really.

That's a good idea.

The logistics worries me a bit: it looks like a major undertaking, considering the 
widespread use of these APIs in 1400+ call sites:

  triton:~/tip> git grep -E '__get_user|__put_user' | grep -vE '\.h:|arch/' | wc -l
  1086
  triton:~/tip> git grep -E '__get_user|__put_user' arch/x86/ arch/arm64/ | wc -l
  354

... affecting 93 files:

  triton:~/tip> git grep -lE '__get_user|__put_user' | grep -vE '\.h:|arch/' | wc -l
  70
  triton:~/tip> git grep -lE '__get_user|__put_user' arch/x86/ arch/arm64/ | wc -l
  23

(Assuming my grep-fu is strong enough.)

Which is probably more than a hundred commits until we get rid of all 
double-underscore uses?

Thanks,

	Ingo
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