kasan: docs: update ignoring accesses section

Update the "Ignoring accesses" section in KASAN documentation:

 - Mention __no_sanitize_address/noinstr.

 - Mention kasan_disable/enable_current().

 - Mention kasan_reset_tag()/page_kasan_tag_reset().

 - Readability and punctuation clean-ups.

Link: https://lkml.kernel.org/r/4531ba5f3eca61f6aade863c136778cc8c807a64.1615559068.git.andreyknvl@google.com
Signed-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

@@ -377,12 +377,18 @@ Ignoring accesses
 ~~~~~~~~~~~~~~~~~
 
 Software KASAN modes use compiler instrumentation to insert validity checks.
-Such instrumentation might be incompatible with some part of the kernel, and
-therefore needs to be disabled. To disable instrumentation for specific files
-or directories, add a line similar to the following to the respective kernel
+Such instrumentation might be incompatible with some parts of the kernel, and
+therefore needs to be disabled.
+
+Other parts of the kernel might access metadata for allocated objects.
+Normally, KASAN detects and reports such accesses, but in some cases (e.g.,
+in memory allocators), these accesses are valid.
+
+For software KASAN modes, to disable instrumentation for a specific file or
+directory, add a ``KASAN_SANITIZE`` annotation to the respective kernel
 Makefile:
 
-- For a single file (e.g. main.o)::
+- For a single file (e.g., main.o)::
 
     KASAN_SANITIZE_main.o := n
 
@@ -390,6 +396,26 @@ Makefile:
 
     KASAN_SANITIZE := n
 
+For software KASAN modes, to disable instrumentation on a per-function basis,
+use the KASAN-specific ``__no_sanitize_address`` function attribute or the
+generic ``noinstr`` one.
+
+Note that disabling compiler instrumentation (either on a per-file or a
+per-function basis) makes KASAN ignore the accesses that happen directly in
+that code for software KASAN modes. It does not help when the accesses happen
+indirectly (through calls to instrumented functions) or with the hardware
+tag-based mode that does not use compiler instrumentation.
+
+For software KASAN modes, to disable KASAN reports in a part of the kernel code
+for the current task, annotate this part of the code with a
+``kasan_disable_current()``/``kasan_enable_current()`` section. This also
+disables the reports for indirect accesses that happen through function calls.
+
+For tag-based KASAN modes (include the hardware one), to disable access
+checking, use ``kasan_reset_tag()`` or ``page_kasan_tag_reset()``. Note that
+temporarily disabling access checking via ``page_kasan_tag_reset()`` requires
+saving and restoring the per-page KASAN tag via
+``page_kasan_tag``/``page_kasan_tag_set``.
 
 Tests
 ~~~~~