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.