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SubjectRe: [git pull] core, x86: make LIST_POISON less deadly

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Mon, 14 Jul 2008, Ingo Molnar wrote:
> > >
> > > Why not use something sane like 0xdead000000000000, which has the high
> > > bit set but very fundamentally isn't a valid pointer, and never will
> > > be? And which is a *lot* more visually obvious too!
> >
> > initially i suggested that too - but such addresses raise a #GP instead
> > of a page fault so their decoding is a bit harder.
>
> But raising a GP is exactly what you want: a PF is an indication that the
> address was actually half-way valid, and will not fault at all on some
> (possibly future) machine.
>
> > We dont do any instruction decoding in #GP handlers to figure out what
> > happened, while in the pagefault case we know which address faulted,
> > etc.
>
> Why would we care? It would be very obvious from the instruction
> disassembly plus the register contents. No need to decode instructions.
>
> > Perhaps we could try to make #GP handlers a bit more informative -
> > although decoding instructions will make things a bit more fragile
> > inevitably.
> >
> > Perhaps make it 0xffffcdead0000000 ?
>
> I'm really not seeing the reason for not just doing it right.

ok. Find the updated pull request below. I've added your Acked-by to the
last commit.

Please pull the latest safe-poison-pointers git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip.git safe-poison-pointers

Thanks,

Ingo

------------------>
Avi Kivity (1):
core, x86: make LIST_POISON less deadly

Ingo Molnar (1):
x86: change LIST_POISON to 0xdead000000000000


arch/x86/Kconfig | 5 +++++
include/linux/poison.h | 10 ++++++++--
2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e0edaaa..0dbd040 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1019,6 +1019,11 @@ config ARCH_MEMORY_PROBE
def_bool X86_64
depends on MEMORY_HOTPLUG

+config ILLEGAL_POINTER_VALUE
+ hex
+ default 0 if X86_32
+ default 0xdead000000000000 if X86_64
+
source "mm/Kconfig"

config HIGHPTE
diff --git a/include/linux/poison.h b/include/linux/poison.h
index 9f31683..0d105a5 100644
--- a/include/linux/poison.h
+++ b/include/linux/poison.h
@@ -1,14 +1,20 @@
#ifndef _LINUX_POISON_H
#define _LINUX_POISON_H

+#ifdef CONFIG_ILLEGAL_POINTER_VALUE
+#define POISON_POINTER_DELTA CONFIG_ILLEGAL_POINTER_VALUE
+#else
+#define POISON_POINTER_DELTA 0L
+#endif
+
/********** include/linux/list.h **********/
/*
* These are non-NULL pointers that will result in page faults
* under normal circumstances, used to verify that nobody uses
* non-initialized list entries.
*/
-#define LIST_POISON1 ((void *) 0x00100100)
-#define LIST_POISON2 ((void *) 0x00200200)
+#define LIST_POISON1 ((void *) 0x00100100 + POISON_POINTER_DELTA)
+#define LIST_POISON2 ((void *) 0x00200200 + POISON_POINTER_DELTA)

/********** include/linux/timer.h **********/
/*

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