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. Because the origin of your bug is likely a NULL pointer. 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"); -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
