linux-kernel - [PATCH v2 10/11] kasan: docs: update ignoring accesses section

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Date:   Fri, 12 Mar 2021 15:24:33 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: [PATCH v2 10/11] 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.

Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>

---

Changes in v1->v2:
- Mention __no_sanitize_address/noinstr.
- Reword the whole section to make it clear which method works for which
  mode.
---
 Documentation/dev-tools/kasan.rst | 34 +++++++++++++++++++++++++++----
 1 file changed, 30 insertions(+), 4 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index d0c1796122df..5749c14b38d0 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -368,12 +368,18 @@ 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
 
@@ -381,6 +387,26 @@ 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
 ~~~~~
-- 
2.31.0.rc2.261.g7f71774620-goog