linux-kernel - [PATCH 09/11] kasan: docs: update shadow memory section

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Date:   Thu, 11 Mar 2021 22:37:21 +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 09/11] kasan: docs: update shadow memory section

Update the "Shadow memory" section in KASAN documentation:

- Rearrange the introduction paragraph do it doesn't give a
  "KASAN has an issue" impression.
- Update the list of architectures with vmalloc support.
- Punctuation, readability, and other minor clean-ups.

Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
---
 Documentation/dev-tools/kasan.rst | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
index f5c746a475c1..2b61d90e136f 100644
--- a/Documentation/dev-tools/kasan.rst
+++ b/Documentation/dev-tools/kasan.rst
@@ -304,14 +304,11 @@ checking gets disabled.
 Shadow memory
 -------------
 
-The kernel maps memory in a number of different parts of the address
-space. This poses something of a problem for KASAN, which requires
-that all addresses accessed by instrumented code have a valid shadow
-region.
-
-The range of kernel virtual addresses is large: there is not enough
-real memory to support a real shadow region for every address that
-could be accessed by the kernel.
+The kernel maps memory in several different parts of the address space.
+The range of kernel virtual addresses is large: there is not enough real
+memory to support a real shadow region for every address that could be
+accessed by the kernel. Therefore, KASAN only maps real shadow for certain
+parts of the address space.
 
 By default
 ~~~~~~~~~~
@@ -323,10 +320,9 @@ page is mapped over the shadow area. This read-only shadow page
 declares all memory accesses as permitted.
 
 This presents a problem for modules: they do not live in the linear
-mapping, but in a dedicated module space. By hooking in to the module
-allocator, KASAN can temporarily map real shadow memory to cover
-them. This allows detection of invalid accesses to module globals, for
-example.
+mapping but in a dedicated module space. By hooking into the module
+allocator, KASAN temporarily maps real shadow memory to cover them.
+This allows detection of invalid accesses to module globals, for example.
 
 This also creates an incompatibility with ``VMAP_STACK``: if the stack
 lives in vmalloc space, it will be shadowed by the read-only page, and
@@ -337,9 +333,10 @@ CONFIG_KASAN_VMALLOC
 ~~~~~~~~~~~~~~~~~~~~
 
 With ``CONFIG_KASAN_VMALLOC``, KASAN can cover vmalloc space at the
-cost of greater memory usage. Currently this is only supported on x86.
+cost of greater memory usage. Currently, this is supported on x86,
+riscv, s390, and powerpc.
 
-This works by hooking into vmalloc and vmap, and dynamically
+This works by hooking into vmalloc and vmap and dynamically
 allocating real shadow memory to back the mappings.
 
 Most mappings in vmalloc space are small, requiring less than a full
@@ -358,10 +355,10 @@ memory.
 
 To avoid the difficulties around swapping mappings around, KASAN expects
 that the part of the shadow region that covers the vmalloc space will
-not be covered by the early shadow page, but will be left
-unmapped. This will require changes in arch-specific code.
+not be covered by the early shadow page but will be left unmapped.
+This will require changes in arch-specific code.
 
-This allows ``VMAP_STACK`` support on x86, and can simplify support of
+This allows ``VMAP_STACK`` support on x86 and can simplify support of
 architectures that do not have a fixed module region.
 
 For developers
-- 
2.31.0.rc2.261.g7f71774620-goog