On  9.03.2017 03:58, Theodore Ts'o wrote:
> On Tue, Mar 07, 2017 at 10:40:53PM +0200, Nikolay Borisov wrote:
>> So this is wrong, the reason why the issues seemed fix is because I
>> switched my compiler to version 5.4.0. So this manifests only if I'm
>> using gcc 4.7.4. With the pr_info added here is the output of a boot. So
>> there are multiple invocations of ext4_ext_map_blocks and the freeing,
>> including with the address being used in subsequent kasan reports :
>> ffff88006ae8fdb0
> 
> Can you help bisect this, then?  I'm using Debian Testing, and the
> default gcc is gcc 6.3.0.  I'm currently forcing the use of gcc 5.4.1
> because I was running into problems with gcc 6.x a while back.  (TBH,
> I was thinking about trying to see if gcc 6.3 was stable for kernel
> compiles when I had some spare time.)  But I don't have access to
> *any* gcc 4.x on my development system, and I don't think I've tried
> using gcc 4.x in a long, Long, LONG time.
> 
> I'm currently kicking off a test run using 5.4.1 with KASAN enabled to
> see if I can trigger it myself.  Can you send me a copy of your
> .config so I can see what else might be interesting with your config?
> (e.g., SLAB vs SLUB, etc.)

Attached the config. FUrther debugging and talking with the kasan
developers I think this actually might be a kasan problem when used with
an old compiler.  I bisected this all the way to 1771c6e1a567ea0ba2,
which is the commit introducing the user access instrumentation. Here is
a mail thread where I confirmed that this might be a kasan issue :
https://lkml.org/lkml/2017/3/8/69

What I believe is happening is that the manual checks inserted in user
access code misses some context information due to instrumentation not
inserted by the compiler. Kasan gets confused as a result, hence the
warnings.


> 
> Thanks,
> 
> 	     	     	       - Ted
>