2010/11/22 Jakub Kiciński <moorray@xxxxx>
Hi,
Dnia 19-11-2010 o 06:17:53 Venkatram Tummala <venkatram867@xxxxxxxxx> napisał(a):
2. For the kernel code, is it feasible to the use the user stack? WhyThe answer is Yes, you could. But it would be pretty messy & inconvenient.
do we bother to switch to the kernel stack?
We just don't do it in the linux kernel atleast on x86. Kernel Data Segment
& User Data Segment is different. I guess you could just map the user space
stack in the kernel address space too & use it. Using two seperate stacks is
just more efficient & convenient.
Wouldn't it be a security bug to use the same stack for both? Kernel function's parameters and auto variables would be still sitting above (well, in x86 under ;) stack pointer. Not sure though if attacker could find anything interesting there...
I don't think so. Kernel function's parameters & auto variables on the stack only live until the lifetime of the procedure. If we were using just one stack for both user & kernel, when we make a switch (1) user -> kernel & then (2) kernel->user, the stack would be in the state it was before (1). Its a stack. If procedure A calls Procedure B in single shared address space, they share the stack. When procedure B returns, there is nothing to poke around for Procedure A about Procedure B on the stack.
Venkatram Tummala
Regards,
moorray
