kasan: remove use after scope bugs detection.

Use after scope bugs detector seems to be almost entirely useless for
the linux kernel.  It exists over two years, but I've seen only one
valid bug so far [1].  And the bug was fixed before it has been
reported.  There were some other use-after-scope reports, but they were
false-positives due to different reasons like incompatibility with
structleak plugin.

This feature significantly increases stack usage, especially with GCC <
9 version, and causes a 32K stack overflow.  It probably adds
performance penalty too.

Given all that, let's remove use-after-scope detector entirely.

While preparing this patch I've noticed that we mistakenly enable
use-after-scope detection for clang compiler regardless of
CONFIG_KASAN_EXTRA setting.  This is also fixed now.

[1] http://lkml.kernel.org/r/<20171129052106.rhgbjhhis53hkgfn@wfg-t540p.sh.intel.com>

Link: http://lkml.kernel.org/r/20190111185842.13978-1-aryabinin@virtuozzo.com
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Will Deacon <will.deacon@arm.com>		[arm64]
Cc: Qian Cai <cai@lca.pw>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 0 insertions, 24 deletions

diff --git a/lib/test_kasan.c b/lib/test_kasan.c
index 51b78405bf24..7de2702621dc 100644
--- a/lib/test_kasan.c
+++ b/lib/test_kasan.c
@@ -480,29 +480,6 @@ static noinline void __init copy_user_test(void)
 	kfree(kmem);
 }
 
-static noinline void __init use_after_scope_test(void)
-{
-	volatile char *volatile p;
-
-	pr_info("use-after-scope on int\n");
-	{
-		int local = 0;
-
-		p = (char *)&local;
-	}
-	p[0] = 1;
-	p[3] = 1;
-
-	pr_info("use-after-scope on array\n");
-	{
-		char local[1024] = {0};
-
-		p = local;
-	}
-	p[0] = 1;
-	p[1023] = 1;
-}
-
 static noinline void __init kasan_alloca_oob_left(void)
 {
 	volatile int i = 10;
@@ -682,7 +659,6 @@ static int __init kmalloc_tests_init(void)
 	kasan_alloca_oob_right();
 	ksize_unpoisons_memory();
 	copy_user_test();
-	use_after_scope_test();
 	kmem_cache_double_free();
 	kmem_cache_invalid_free();
 	kasan_memchr();