[coreboot] libpayload alloc() gcc 4.6.2 bug?

[Marc Jones]

On Wed, Feb 29, 2012 at 5:31 PM, Carl-Daniel Hailfinger
<c-d.hailfinger.devel.2006 at gmx.net> wrote:
> Am 29.02.2012 08:39 schrieb Patrick Georgi:
>> Am 28.02.2012 23:06, schrieb Marc Jones:
>>> I found this bug building tint with libpayload. libpayload is built
>>> with defconfig and using the same coreboot crosstools gcc. The bug
>>> happens in the first call to alloc() when the first header of the
>>> first region is installed. The header memory location is checked,
>>> found to be 0, and then loaded with the header. The bug is that the
>>> original value of the location is used after the memory was updated.
>>> It should have been reloaded. It is pretty easy to see in the
>>> disassembly below.
>> workaround: mark setup() __attribute__((noinline))
>>
>> The proper fix is to clean up the various casts so the aliasing based
>> optimizations in gcc do the right thing.
>
> Can't you use __attribute__((may_alias)) for the affected variables?
>
> Regards,
> Carl-Daniel
>
> --
> http://www.hailfinger.org/
>

i think that I have resolved the issue by making the pointers
volatile. I wanted to get some comments here before I push it to
gerrit.

The asm generated:

  10b7d7:	8b 15 a0 5f 11 00    	mov    0x115fa0,%edx
  10b7dd:	81 e2 00 00 00 a8    	and    $0xa8000000,%edx
  10b7e3:	81 fa 00 00 00 a8    	cmp    $0xa8000000,%edx
  10b7e9:	74 0f                	je     10b7fa <alloc+0x3a>
  10b7eb:	25 fc ff ff 00       	and    $0xfffffc,%eax
  10b7f0:	0d 00 00 00 aa       	or     $0xaa000000,%eax
  10b7f5:	a3 a0 5f 11 00       	mov    %eax,0x115fa0
  10b7fa:	b8 a0 5f 11 00       	mov    $0x115fa0,%eax
  10b7ff:	90                   	nop
  10b800:	8b 10                	mov    (%eax),%edx
  10b802:	89 d1                	mov    %edx,%ecx


Marc

-- 
http://se-eng.com
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-Make-libpaylod-alloc-memory-pointers-volatile.patch
Type: text/x-patch
Size: 2924 bytes
Desc: not available
URL: <http://www.coreboot.org/pipermail/coreboot/attachments/20120301/8552d40b/attachment.patch>