Re: [PATCH 16/17] x86-64: Remove the PDA

From: Brian Gerst
Date: Sun Jan 18 2009 - 02:46:31 EST

On Sat, Jan 17, 2009 at 11:52 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
> Hello,
> Brian Gerst wrote:
>> Now that the PDA is empty except for the stack canary, it can be removed.
>> The irqstack is moved to the start of the per-cpu section. If the stack
>> protector is enabled, the canary overlaps the bottom 48 bytes of the irqstack
>> on SMP. On UP it is a seperate variable, since it is the only thing referenced
>> via %gs.
> Eh... I don't know. Locating stack canary at hard 40byte offset is a
> dirty thing to do one way or another. I kind of like doing it
> directly in the linker script as it makes the dirty nature more
> obvious and doesn't require hunting down the definition in the first
> section.
> How about something like the following?
> #define CANARY_OFFSET 40
> #define CANARY_SIZE 8
> DECLARE_PER_CPU(unsigned long, stack_canary);
> and in linker script,
> per_cpu__stack_canary = __per_cpu_start + CANARY_OFFSET;

The thing I don't like about the prealloc method is that it puts the
page-aligned variables at the end. This leaves a gap which is
unavailable for dynamic allocations. Stealing 48 bytes from the
bottom of the irqstack (which is 16k) keeps the page-aligned section
at the start. It's really no different than how the thread_info
structure sits at the bottom of the process stack.

How about something like:
union irq_stack_union {
char irq_stack[IRQSTACKSIZE];
struct {
char pad[40];
unsigned long stack_canary;

That documents the overlay better, and avoids having to touch the linker script.

Brian Gerst
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