kasan: docs: update GENERIC implementation details section

Update the "Implementation details" section for generic KASAN:

 - Don't mention kmemcheck, it's not present in the kernel anymore.

 - Don't mention GCC as the only supported compiler.

 - Update kasan_mem_to_shadow() definition to match actual code.

 - Punctuation, readability, and other minor clean-ups.

Link: https://lkml.kernel.org/r/f2f35fdab701f8c709f63d328f98aec2982c8acc.1615559068.git.andreyknvl@google.comSigned-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/dev-tools/kasan.rst

@@ -209,12 +209,11 @@ Implementation details
 Generic KASAN
 ~~~~~~~~~~~~~
 
-From a high level perspective, KASAN's approach to memory error detection is
-similar to that of kmemcheck: use shadow memory to record whether each byte of
-memory is safe to access, and use compile-time instrumentation to insert checks
-of shadow memory on each memory access.
+Software KASAN modes use shadow memory to record whether each byte of memory is
+safe to access and use compile-time instrumentation to insert shadow memory
+checks before each memory access.
 
-Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (e.g. 16TB
+Generic KASAN dedicates 1/8th of kernel memory to its shadow memory (16TB
 to cover 128TB on x86_64) and uses direct mapping with a scale and offset to
 translate a memory address to its corresponding shadow address.
 
@@ -223,23 +222,23 @@ address::
 
     static inline void *kasan_mem_to_shadow(const void *addr)
     {
-	return ((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
+	return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT)
 		+ KASAN_SHADOW_OFFSET;
     }
 
 where ``KASAN_SHADOW_SCALE_SHIFT = 3``.
 
 Compile-time instrumentation is used to insert memory access checks. Compiler
-inserts function calls (__asan_load*(addr), __asan_store*(addr)) before each
-memory access of size 1, 2, 4, 8 or 16. These functions check whether memory
-access is valid or not by checking corresponding shadow memory.
+inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before
+each memory access of size 1, 2, 4, 8, or 16. These functions check whether
+memory accesses are valid or not by checking corresponding shadow memory.
 
-GCC 5.0 has possibility to perform inline instrumentation. Instead of making
-function calls GCC directly inserts the code to check the shadow memory.
-This option significantly enlarges kernel but it gives x1.1-x2 performance
-boost over outline instrumented kernel.
+With inline instrumentation, instead of making function calls, the compiler
+directly inserts the code to check shadow memory. This option significantly
+enlarges the kernel, but it gives an x1.1-x2 performance boost over the
+outline-instrumented kernel.
 
-Generic KASAN is the only mode that delays the reuse of freed object via
+Generic KASAN is the only mode that delays the reuse of freed objects via
 quarantine (see mm/kasan/quarantine.c for implementation).
 
 Software tag-based KASAN