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SubjectRe: [PATCH 1/8] kasan,x86: Frob kasan_report() in an exception
On Wed, Mar 06, 2019 at 06:14:51PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 06, 2019 at 02:13:47PM +0100, Peter Zijlstra wrote:
> > +static __always_inline unsigned long smap_save(void)
> > +{
> > + unsigned long flags = arch_local_save_flags();
> > + clac();
> > + return flags;
> > +}
> > +
> > +static __always_inline void smap_restore(unsigned long flags)
> > +{
> > + arch_local_irq_restore(flags);
> > +}
>
> ARGH; the bloody paravirt me harder nonsense makes that pvops calls.
>
> And that (obviously) explodes.. Anybody got any clue why that Xen
> trainwreck wants to paravirt: "PUSHF;POP" and "PUSH;POPF" !?
>
> Also; I suppose I can ALTERNATIVE the whole thing, because Xen will not
> be having SMAP in the first place I suppose.

The below seems to 'work'.

--- a/arch/x86/include/asm/smap.h
+++ b/arch/x86/include/asm/smap.h
@@ -46,8 +46,6 @@

#ifdef CONFIG_X86_SMAP

-#include <asm/irqflags.h>
-
static __always_inline void clac(void)
{
/* Note: a barrier is implicit in alternative() */
@@ -62,14 +60,19 @@ static __always_inline void stac(void)

static __always_inline unsigned long smap_save(void)
{
- unsigned long flags = arch_local_save_flags();
- clac();
+ unsigned long flags;
+
+ asm volatile (ALTERNATIVE("", "pushf; pop %0; " __stringify(__ASM_CLAC),
+ X86_FEATURE_SMAP)
+ : "=rm" (flags) : : "memory", "cc");
+
return flags;
}

static __always_inline void smap_restore(unsigned long flags)
{
- arch_local_irq_restore(flags);
+ asm volatile (ALTERNATIVE("", "push %0; popf", X86_FEATURE_SMAP)
+ : : "g" (flags) : "memory", "cc");
}

/* These macros can be used in asm() statements */
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