linux-kernel - Re: fs: proc: gpf in find

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Date:	Mon, 22 Dec 2014 13:12:21 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	"davej @mail.xmission.com>> Dave Jones" <davej@...hat.com>
Subject: Re: fs: proc: gpf in find_entry

On 12/22/2014 12:52 PM, Andrey Ryabinin wrote:
> 2014-12-22 18:51 GMT+03:00 Eric W. Biederman <ebiederm@...ssion.com>:
>> These two instructions:
>>>>   11:   4d 85 ff                test   %r15,%r15
>>>>   14:   0f 84 de 01 00 00       je     0x1f8
>>
>> Should prevent a NULL %r15 value from ever reaching the trapping
>> instruction.
> 
> If they were executed, then yes. But I think there was jump from somewhere
> to the instructions below those two.

There is indeed a jump direct to that point, which avoids the %r15 check.

>> What other horrible things does KASAN do to the machine code?
>>
> 
> kasan insert something like following before any memory access:
> 
> s8 *shadow_addr = (add >> 3) + shadow_offset;
> 
> if (unlikely(*shadow_addr))
>      if (unlikely(addr & 7 >= *shadow_addr))
>            report_bug(addr);
> 
> 
> I suspect that Sasha is using kasan along with ubsan.
> In that case generated code much more horrid.

It's not *that* bad :)


Thanks,
Sasha
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