In general, user cannot see kernel memory, but kernel can see user memory. To enforce this, all memory are not to be shared. And this is also enforce at the x86 hardware level - no OS feature is needed (ie, windows/linux all worked in the same way)....the keyword to search is called "stack switching". And kernel have two stack - one for interrupt processing, and another the normal non-interrupt processing. On Thu, Apr 24, 2008 at 5:11 PM, sahlot arvind <asahlot@xxxxxxxxx> wrote: > Why do we have two stacks per process i.e. one user mode and other kernel > mode? > As soon as process enters into kernel mode it starts using kernel mode > stack, but as soon as it comes out of kernel mode stack there is nothing in > the kernel mode stack. So cannot we simply have a single stack i.e. just > user mode stack which can also be used for kernel mode execution of process this violate the first principle mentioned above.....the current CPU may be processing in kernel mode, but another CPU may be processing in userspace mode, same process, and as all the userspace memory are visible to him, he can view the kernel's data. even though (in theory, and not using the hardware stack switching mechanism) it is possible to set the privilege tag for the memory (called DPL, to match the CPU processing privilege - CPL) any CPU running at ring 3 (which is CPL3) cannot see the memory's data assigned at DPL0, but then u will have incur the overhead of doing this FOR EVERY switching between ring0 and ring3. and this attributes cannot be set at the granularity of bytes level - not what is that, but I think it is something like every 0x1000 bytes - so u may end up with lots of memory padding - wasting more memory instead. > after all there wont be any data related to kernel (all would have been not possible....u mean zero out ALL the kernel local variable? then when it come back to kernel again, it will see all its variable zeroed? don't forget that kernel function can BLOCK....and then execution get transfer back to userspace ..... when the kernelspace is not yet completet. > popped off) in the stack when we are back in user mode? > > If you say that its just the stack pointer which is adjusted during pushes > and pops and the stack content remains intact and user can try to > increment/decrement stack pointer in order to access kernel data (which is > stale however) then instead of having two stacks (kernel and user) cannot we > simply zero out the stack area used by kernel before returing to user mode? > and can we save memory by having just one stack? > > Best regards > - A > http://linuxexplained.blogspot.com -- Regards, Peter Teoh -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
