Re: [2/3] mm: fix up some user-visible effects of the stack guard page

[Linus Torvalds]

On Fri, Aug 20, 2010 at 5:54 AM, Ian Campbell <ijc@xxxxxxxxxxxxxx> wrote:
>
> Since we have split the original VMA into 3, shouldn't only the bottom
> one still have VM_GROWSDOWN set? (how can the top two grow down with the
> bottom one in the way?) Certainly it seems wrong to enforce a guard page
> on anything but the bottom VMA (which is what appears to be happening).

Yes, it does seem like we should teach vma splitting to remove
VM_GROWSDOWN on all but the lowest mapping.

> Out of interest how does the guard page interact with processes which do
> alloca(N*PAGE_SIZE)?

It's a guard page, not magic. Some architecture ABI's specify that if
you expand the stack by more than a certain number, you need to touch
a page in between (for example, I think alpha had that rule), because
they don't grow the stack automatically by an arbitrary amount. But
x86 has never had that rule, and you can certainly defeat a guard page
by simply accessing by much more than a page.

As far as I'm concerned, the guard page thing is not - and shouldn't
be thought of - a "hard" feature. If it's needed, it's really a bug in
user space. But given that there are bugs in user space, the guard
page makes it a bit harder to abuse those bugs. But it's about "a bit
harder" rather than anything else.

IOW, it does _not_ make up for user space that willfully does crazy
things, and never will.

                       Linus
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