Marek Vasut marex@denx.de wrote on 2012/10/16 08:31:20:
Dear Graeme Russ,
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.
Ok, I don't mean to reopen this can of worms again ... but what're we
going to
do about this patch?
Skip the idea to protect a page, this is too complicated for a boot loader. Just treat malloc(0) as malloc(1) internally.
Jocke