On Fri, Jun 18, 2010 at 10:39 AM, Frederic Weisbecker <fweisbec@xxxxxxxxx> wrote:
Because the origin of your bug is likely a NULL pointer.On Fri, Jun 18, 2010 at 10:11:13AM +0530, Prasad Joshi wrote:
> Hi All,
>
> I am trying to understand a a kernel oops report. Here are some of the
> fields from the report
>
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000034
> Oops: 0000 [#1] SMP
> Pid: 6478, comm: cp Tainted: P 2.6.31.5-127.fc12.x86_64 #1 Inspiron 1525
> RIP: 0010:[<ffffffff810faac1>] [<ffffffff810faac1>] do_sys_open+0x7a/0x10f
> CR2: 0000000000000034
>
> As I know, when a page fault occurs, the address the program attempted to
> access is stored in the CR2 register. So probably the pointer is pointing to
> address 0034 and is being access.
>
> The BUG string is bit confusing, it says NULL pointer dereference at 0034, I
> know the address 00034 is not valid but why is it interpreted as NULL
> pointer? The NULL pointer as I know should point to address 0.
Such address dereferenced are often the case of accessing a
member of a structure, or an index of an array, which base
address is 0.
So Linux assumes that such very low addresses are an offset
from a NULL pointer. The max threshold to determine this is a
page size (typically 4096).
arch/x86/mm/fault.c:
printk(KERN_ALERT "BUG: unable to handle kernel ");
if (address < PAGE_SIZE)
printk(KERN_CONT "NULL pointer dereference");
else
printk(KERN_CONT "paging request");
Oh I see, thanks Fredric for you reply.
Thanks and Regards,
Prasad
