Re: [patch 19/24] TASK_SIZE is variable.

["David S. Miller" <davem@davemloft.net>]

On Sun, 6 Feb 2005 22:50:20 +0100
Andi Kleen <ak@suse.de> wrote:

> And yes this stuff does matter - i remember i got LM benchmarkable
> improvements in signal latency by optimizing __copy_to_user
> to use optimized inlines for small stores.

Moving access_ok() out-of-line might even improve I-cache access
over what we have today, even with the new min-max check.  The
min-max variables will be in the same cache line in whatever
struct we place them into, so whatever cache miss access_ok() gets
now will also be the same for the min-max version.

This is kind of strange to be arguing about, given that we just
put 4-level page tables into the tree, right?  That regressed
everybody performance wise, even people not using the full
4-level support.  But I have not barked at you about this, I
undersand why it's needed.  And yet you're using lmbench cycle
counting to justify your position against this new verification
scheme.

And it's not just a sparc64 issue.  Sparc64 hardware traps the
access, but it's a bug regardless of platform to try to do user
accesses whilst get_fs()==KERNEL_DS.  All the user has to do is
pass in a valid kernel address and you have a root exploit.  I
mean, do folks really disagree with this?