Subject: Re: random signals kill my processes with -current
To: Bill Studenmund <firstname.lastname@example.org>
From: Jason Thorpe <email@example.com>
Date: 01/27/1997 18:31:49
On Mon, 27 Jan 1997 16:33:23 -0800 (PST)
Bill Studenmund <firstname.lastname@example.org> wrote:
> This idea is different from a guard page in that we don't get a
> screetching halt when we overflow, but would work on processors that would
> freak if they didn't have a stack (ones that lack fall-back stacks).
> Hopefully the overflow is small, so that the buffer protects whatever's
> below it. And hopefully we don't overflow with code which gives the exact
> flag value as a subroutine parameter.
Ok, so I decided to take a look at the original 4.4BSD STACKCHECK
code in the hp300 port (long since gone from NetBSD :-). The comment
at the top of locore.s:
* STACKCHECK enables two types of kernel stack checking:
* 1. stack "overflow". On every clock interrupt we ensure that
* the current kernel stack has not grown into the user struct
* page, i.e. size exceeded UPAGES-1 pages.
* 2. stack "underflow". Before every rte to user mode we ensure
* that we will be exactly at the base of the stack after the
* exception frame has been popped.
* Both checks are performed at splclock since they operate on the
* global temporary stack.
This checking was turned off by default, since it can be kind of
expensive (and since clock interrupts happen 100 times a second,
you want it to be as inexpensive as possible, especially on
something like an hp320 :-)
Jason R. Thorpe email@example.com
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