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. -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
