kasan: remove kasan_unpoison_stack_above_sp_to()

kasan_unpoison_stack_above_sp_to() is defined in kasan code but never
used.  The function was introduced as part of the commit:

   commit 9f7d416c ("kprobes: Unpoison stack in jprobe_return() for KASAN")

... where it was necessary because x86's jprobe_return() would leave
stale shadow on the stack, and was an oddity in that regard.

Since then, jprobes were removed entirely, and as of commit:

  commit 80006dbe ("kprobes/x86: Remove jprobe implementation")

... there have been no callers of this function.

Remove the declaration and the implementation.
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andrey Konovalov <andreyknvl@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Link: http://lkml.kernel.org/r/20200706143505.23299-1-vincenzo.frascino@arm.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kasan.h

@@ -38,7 +38,6 @@ extern void kasan_disable_current(void);
 void kasan_unpoison_shadow(const void *address, size_t size);
 
 void kasan_unpoison_task_stack(struct task_struct *task);
-void kasan_unpoison_stack_above_sp_to(const void *watermark);
 
 void kasan_alloc_pages(struct page *page, unsigned int order);
 void kasan_free_pages(struct page *page, unsigned int order);
@@ -101,7 +100,6 @@ void kasan_restore_multi_shot(bool enabled);
 static inline void kasan_unpoison_shadow(const void *address, size_t size) {}
 
 static inline void kasan_unpoison_task_stack(struct task_struct *task) {}
-static inline void kasan_unpoison_stack_above_sp_to(const void *watermark) {}
 
 static inline void kasan_enable_current(void) {}
 static inline void kasan_disable_current(void) {}

mm/kasan/common.c

@@ -180,21 +180,6 @@ asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
 	kasan_unpoison_shadow(base, watermark - base);
 }
 
-/*
- * Clear all poison for the region between the current SP and a provided
- * watermark value, as is sometimes required prior to hand-crafted asm function
- * returns in the middle of functions.
- */
-void kasan_unpoison_stack_above_sp_to(const void *watermark)
-{
-	const void *sp = __builtin_frame_address(0);
-	size_t size = watermark - sp;
-
-	if (WARN_ON(sp > watermark))
-		return;
-	kasan_unpoison_shadow(sp, size);
-}
-
 void kasan_alloc_pages(struct page *page, unsigned int order)
 {
 	u8 tag;