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SubjectRe: [PATCH 1/5] lguest host feedback tidyups
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On Sun, 2007-05-13 at 19:39 +0100, Al Viro wrote:
> On Fri, May 11, 2007 at 11:19:14AM +1000, Rusty Russell wrote:
> > @@ -218,7 +218,7 @@ u32 lgread_u32(struct lguest *lg, u32 ad
> >
> > /* Don't let them access lguest binary */
> > if (!lguest_address_ok(lg, addr, sizeof(val))
> > - || get_user(val, (u32 __user *)addr) != 0)
> > + || get_user(val, (__force u32 __user *)addr) != 0)
> > kill_guest(lg, "bad read address %u", addr);
> > return val;
>
> *Ahem*
>
> What kind of address are we really getting there? IOW, where does it
> ultimately come from?

Hi Al,

This patch has been superseded. But to clarify, the address generally
comes from a guest register. My confusion came from sparse warnings on
the above code like the following:

warning: cast removes address space of expression

I inserted __force, but it's actually caused by the "addr" being a
"u32": if it's an "unsigned long" sparse doesn't warn. This is a win
anyway: although the code is i386-specific at the moment, that will
change. I prefer to use u32 rather than unsigned long to declare
registers (eg. if I ever wanted to support 32 bit guests on a 64 bit
host), but that's not even a consideration at this stage.

> > lock_cpu_hotplug();
> > if (cpu_has_pge) { /* We have a broader idea of "global". */
> > cpu_had_pge = 1;
> > - on_each_cpu(adjust_pge, 0, 0, 1);
> > + on_each_cpu(adjust_pge, (void *)0, 0, 1);
>
> That's called NULL...

Yes, but this is clearer. Here's adjust_pge:

static void adjust_pge(void *on)
{
if (on)
write_cr4(read_cr4() | X86_CR4_PGE);
else
write_cr4(read_cr4() & ~X86_CR4_PGE);
}
And here's the two calls to it:

on_each_cpu(adjust_pge, (void *)0, 0, 1);
...
on_each_cpu(adjust_pge, (void *)1, 0, 1);

> > case LHCALL_LOAD_TLS:
> > - guest_load_tls(lg, (struct desc_struct __user*)regs->edx);
> > + guest_load_tls(lg,
> > + (__force struct desc_struct __user*)regs->edx);
>
> Umm... That's borderline OK, but...

Yeah, gone in replacement patch.

> > static void push_guest_stack(struct lguest *lg, u32 __user **gstack, u32 val)
> > {
> > - lgwrite_u32(lg, (u32)--(*gstack), val);
> > + lgwrite_u32(lg, (__force u32)--(*gstack), val);
> > }
>
> Now, _that_ is just plain dumb. Why not declare that lgwrite_u32() as taking
> u32 __user * as argument and kill the casts?

Last I tried, this turns out to create even more casts.

> > - lg->regs->esp = (u32)gstack + lg->page_offset;
> > + lg->regs->esp = (__force u32)gstack + lg->page_offset;
>
> Yuck. Cast to unsigned long (or uintptr_t), please. In this case it is
> legitimate.

Indeed, replacement patch uses unsigned long.

Thanks,
Rusty.

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