On Fri, May 2, 2014 at 11:12 PM, H. Peter Anvin <h...@zytor.com> wrote: > On 05/02/2014 09:32 PM, Linus Torvalds wrote: >> >> At least as a proof-of-concept, having a code sequence in user mode >> trampoline that does >> >> popq %rsi >> popq %r11 >> retq $128 >> >> and building up a stack in user space at '%rsp-128' that has the >> values or rsi/r11/rip should allow us to use 'sysret'. Hmm? > > That would be a security hole if another userspace thread could muck > with the stack.
No, all of the above is in user space, and the pre-restore register values for rsi/r11/rip/rsp are all user space values (just not the right ones for the "real" return point). So no security issue. Now, replacing "iret" with "sysret + user-space trampoline" doesn't work in general (it gets RF wrong, for example, so it's useless for single-stepping and breakpoint handling), but I was more thinking that it would be an interesting way to see what the performance impact of a faster iret would be. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/