[PATCH] ARM: poison initmem when it is freed

Stephen Boyd sboyd at codeaurora.org
Wed Jul 6 16:55:54 EDT 2011

On 07/06/2011 01:34 PM, Russell King - ARM Linux wrote:
> On Tue, Jul 05, 2011 at 04:34:39PM -0700, Stephen Boyd wrote:
>> On 07/05/2011 12:48 PM, Nicolas Pitre wrote:
>>> On Tue, 5 Jul 2011, Russell King - ARM Linux wrote:
>>>> On Tue, Jul 05, 2011 at 03:17:33PM -0400, Nicolas Pitre wrote:
>>>>> On Tue, 5 Jul 2011, Russell King - ARM Linux wrote:
>>>>>> When the initmem is freed, we can no longer rely on its contents.  In
>>>>>> lightly loaded systems, this memory may persist for some time, making
>>>>>> it harder discover run-time issues (caused by the build warnings being
>>>>>> ignored.)
>>>>>> Poison the initmem at the point where it is freed to encourage run-time
>>>>>> problems when initmem is dereferenced as an aid to finding such problems.
>>>>>> Signed-off-by: Russell King <rmk+kernel at arm.linux.org.uk>
>>>>> The default poison doesn't appear to be a judicious choice for ARM.
>>>>> include/linux/poison.h:#define POISON_FREE_INITMEM      0xcc
>>>>>    0:   cccccccc        stclgt  12, cr12, [ip], {204}   ; 0xcc
>>>>> So if the gt condition is false this will execute nops until it falls 
>>>>> out of the initmem section.  Would be nicer if a fault could be 
>>>>> generated right at the accessed address which could be looked up.
>>>> Have you tried to find a byte-based poison value which would fault
>>>> yet still cause a pointer dereference?  You're limited to 0xeN on
>>>> ARM, of which there's almost nothing to chose from:
>>>>    0:   e0e0e0e0        rsc     lr, r0, r0, ror #1
>>>>    4:   e1e1e1e1        mvn     lr, r1, ror #3
>>>>    8:   e2e2e2e2        rsc     lr, r2, #536870926      ; 0x2000000e
>>>>    c:   e3e3e3e3        mvn     lr, #-1946157053        ; 0x8c000003
>>>>   10:   e4e4e4e4        strbt   lr, [r4], #1252
>>>>   14:   e5e5e5e5        strb    lr, [r5, #1509]!
>>>>   18:   e6e6e6e6        strbt   lr, [r6], r6, ror #13
>>>>   1c:   e7e7e7e7        strb    lr, [r7, r7, ror #15]!
>>>>   20:   e8e8e8e8        stmia   r8!, {r3, r5, r6, r7, fp, sp, lr, pc}^
>>>>   24:   e9e9e9e9        stmib   r9!, {r0, r3, r5, r6, r7, r8, fp, sp, lr, pc}^
>>>>   28:   eaeaeaea        b       0xffababd8
>>>>   2c:   ebebebeb        bl      0xffafafe0
>>>>   30:   ecececec        stcl    12, cr14, [ip], #944
>>>>   34:   edededed        stcl    13, cr14, [sp, #948]!
>>>>   38:   eeeeeeee        cdp     14, 14, cr14, cr14, cr14, {7}
>>>>   3c:   efefefef        svc     0x00efefef
>>>> 0xefefefef looks to be about the best alternative.
>>> Right.  Does it have to be a byte?  Having a word (or half-word if 
>>> Thumb2) would be much more convenient.
>>>> It then brings up whether POISON_FREE_INITMEM should be changed or not,
>>>> as 0xcc is the expected value for this at the moment.
>>> I would think that this should be a per architecture value to actually 
>>> be useful.
>> Didn't I already post this patch about 6 months ago?
>> https://lkml.org/lkml/2011/1/11/1
>> Here it is, the only downside I see is the memset isn't really efficient
>> as the assembler optimized one.
> Ok, let's do it your way...
> But, do we need to do it page by page?  Can we not have a function which
> does the poisioning, and we just pass the __init_begin/__init_end and tcm
> start/end stuff to?

Should it include the initrd too? At least x86 poisons that memory but I
don't know who would be using that incorrectly.

How about a free_init_area() function which calls free_area() after
poisoning the memory?

Sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.

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