[PATCH] arm: mm: Poison freed init memory

Russell King - ARM Linux linux at arm.linux.org.uk
Thu Jan 6 04:07:41 EST 2011

On Wed, Jan 05, 2011 at 09:25:55PM -0800, Stephen Boyd wrote:
> On 01/05/2011 12:26 PM, Russell King - ARM Linux wrote:
> > On Wed, Jan 05, 2011 at 11:47:25AM -0800, Stephen Boyd wrote:
> >> Poisoning __init marked memory can be useful when tracking down
> >> obscure memory corruption bugs. When a pointer is 0xCCCCCCCC in an
> >
> > That's a bad idea for a value.  With a 3GB page offset and 256MB or
> > more memory, accesses to such an address will always succeed.
> >
> > There's two things to be considered when selecting a possible poison
> > value:
> >
> > 1. what value is guaranteed to provoke an undefined instruction exception?
> > 2. what value when used as an address and dereferenced is mostly always
> >    going to abort?
> >
> > 1 for ARM mode implies an 0xe7fXXXfX value.  For Thumb mode 0xdeXX.  We
> > use this space for breakpoints.
> >
> > 2 unfortunately depends on the platform. 
> A coworker proposed we use a SWI instruction. We could do that if the
> poison is 0xEF and then do something in the SWI handler where that
> number causes us to blow up?

Doesn't work with EABI - the comment field in the SWI instruction is
ignored on EABI.

> If I'm following correctly, point 1 is about __init functions and point
> 2 is about __initdata. I'm more concerned about __initdata because
> __init functions called from non __init marked functions are usually
> caught with section mismatch checks. Also, if we're jumping to
> 0xCCCCCCCC we're probably not in the text section of the kernel with a

But, as I pointed out, you don't know that 0xCCCCCCCC isn't a valid
address _and_ on modern platforms it won't fault.  So it's pointless
to use it as a poison value.

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