kasan: show gcc version requirements in Kconfig and Documentation

The documentation shows a need for gcc > 4.9.2, but it's really >=.  The
Kconfig entries don't show require versions so add them.  Correct a
latter/later typo too.  Also mention that gcc 5 required to catch out of
bounds accesses to global and stack variables.
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrey Ryabinin <a.ryabinin@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/kasan.txt

@@ -9,7 +9,9 @@ a fast and comprehensive solution for finding use-after-free and out-of-bounds
 bugs.
 
 KASan uses compile-time instrumentation for checking every memory access,
-therefore you will need a certain version of GCC > 4.9.2
+therefore you will need a gcc version of 4.9.2 or later. KASan could detect out
+of bounds accesses to stack or global variables, but only if gcc 5.0 or later was
+used to built the kernel.
 
 Currently KASan is supported only for x86_64 architecture and requires that the
 kernel be built with the SLUB allocator.
@@ -23,8 +25,8 @@ To enable KASAN configure kernel with:
 
 and choose between CONFIG_KASAN_OUTLINE and CONFIG_KASAN_INLINE. Outline/inline
 is compiler instrumentation types. The former produces smaller binary the
-latter is 1.1 - 2 times faster. Inline instrumentation requires GCC 5.0 or
-latter.
+latter is 1.1 - 2 times faster. Inline instrumentation requires a gcc version
+of 5.0 or later.
 
 Currently KASAN works only with the SLUB memory allocator.
 For better bug detection and nicer report, enable CONFIG_STACKTRACE and put

lib/Kconfig.kasan

@@ -10,8 +10,11 @@ config KASAN
 	help
 	  Enables kernel address sanitizer - runtime memory debugger,
 	  designed to find out-of-bounds accesses and use-after-free bugs.
-	  This is strictly debugging feature. It consumes about 1/8
-	  of available memory and brings about ~x3 performance slowdown.
+	  This is strictly a debugging feature and it requires a gcc version
+	  of 4.9.2 or later. Detection of out of bounds accesses to stack or
+	  global variables requires gcc 5.0 or later.
+	  This feature consumes about 1/8 of available memory and brings about
+	  ~x3 performance slowdown.
 	  For better error detection enable CONFIG_STACKTRACE,
 	  and add slub_debug=U to boot cmdline.
 
@@ -40,6 +43,7 @@ config KASAN_INLINE
 	  memory accesses. This is faster than outline (in some workloads
 	  it gives about x2 boost over outline instrumentation), but
 	  make kernel's .text size much bigger.
+	  This requires a gcc version of 5.0 or later.
 
 endchoice