On 04/03/2012 09:35 AM, Graeme Russ wrote: > Hi Jocke > And I really need to check, but I have a sneaking suspicion that as the > code currently stands in U-Boot/x86 dereferencing a NULL pointer won't > cause an exception. In x86, U-Boot configures all protected mode segments > to be 4GB starting at physical address 0x00000000 with no virtual address > translation. Accessing physical address 0x00000000 is just as valid as > accessing 0x00000001 (or any other address). > > Now if I set segments to start at 0x00000002 then I can trap a segmentation > fault for accesses to 0x00000000 (NULL) and 0x00000001 (malloc(0) pointer) > > That will mean that U-Boot cannot ever access those two bytes of memory, > but I doubt that I would ever want to. And I will need to set the segments > to base address 0x00000000 before jumping into Linux...
OK, this is not as easy as it sounds. Detecting NULL pointer dereferences will involve enabling paging[1] which is something I really do not want to do in U-Boot. Flat Protected Mode with a 4GB linear map is perfectly fit for purpose, and that is how the Linux kernel expects things to be configured so it will be a major PITA to change. In short, returning non-NULL from malloc(0) and expecting a CPU exception when it is de-referenced is not going to fly. If we choose this path, at least put a debug() statement in to warn when malloc(0) is called. Regards, Graeme [1] Apparently the way do do it is to reserve the entire first 4kB page and mark it as 'not-present' so any access causes a page-fault. _______________________________________________ U-Boot mailing list U-Boot@lists.denx.de http://lists.denx.de/mailman/listinfo/u-boot
