Re: [PATCH 2.6.21.1] i386: save registers before intra-privilege syscall

H. Peter Anvin wrote:
> Sticking kernel mode values in those fields would add no value, except
> as a poison (since %ss == KERNEL_DS and would cause a #GP(0) if it ever
> reached IRET.)  If anything, those fields should be pushed as zero or
> some other poison bits.  That would be slightly better than what's there
> now, which is whatever garbage happens to be on the stack.  Pushing the
> kernel SS:ESP is just plain wrong (not to mention that the way you do it
> doesn't even produce the right value for ESP -- you'd have to save away
> ESP before you push SS.)

That's true. The xss and esp fields of the pt_regs struct always contain
either garbage (for interrupts occuring while in kernel mode) or
user-mode values (for interrupts occuring while in user mode).
So, filling these fields with kernel-mode values indeed doesn't make
much sense.

Allocating space on the stack and poisoning those values would make
sense, though, so I will modify the patch accordingly and resend it.

Thank you for your feedback!

- Philipp Kohlbecher