4k/8k stacks [LWN.net]

4k/8k stacks

Posted May 8, 2008 23:12 UTC (Thu) by kbob (guest, #1770)
Parent article: The last things through the 2.6.26 merge window

I'm asking this from a position of ignorance -- I haven't looked at kernel code in a long
time.  But wouldn't it be possible to allocate 4k stacks with guard pages between them, thus
(a) saving memory and (b) generating a loud exception on stack overflow?

Or is there some kind of overhead in leaving holes into the kernel address space that I'm not
aware of?

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4k/8k stacks

Posted May 9, 2008 0:48 UTC (Fri) by jmspeex (subscriber, #51639) [Link]

I believe we're talking non-virtual memory, so an overflow would just end up on the next
physical page with no way to detect it. Or maybe I'm just talking through my ass (I'm not a
kernel dev).

4k/8k stacks

Posted May 9, 2008 0:57 UTC (Fri) by corbet (editor, #1) [Link]

All memory is virtual memory when the kernel is running. But kernel-space addresses are a sort of special case, in that they are mapped as one big page using a single page table entry. So it's not possible to insert inaccessible guard pages after the kernel stacks - at least, not without dropping the big mapping, which would have a significant performance cost.

4k/8k stacks

Posted May 15, 2008 11:37 UTC (Thu) by cras (guest, #7000) [Link]

Maybe a small (32bit?) canary at the end of the stack would help a bit? Although if an
overflow 
happens I guess by the time it's noticed the kernel might have crashed already.

4k/8k stacks

Posted May 15, 2008 14:01 UTC (Thu) by mcortese (guest, #52099) [Link]

Kernel address space maps 1-to-1 to physical memory (at least for less than 1 GiB of RAM). The argument is 1 page vs. 2 pages of stack. Having just 1 page of stack plus 1 guard page just doesn't save anything.