[09/11] kasan: docs: update shadow memory section

Commit Message

Andrey Konovalov March 11, 2021, 9:37 p.m. UTC

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 <[e-mail]>
---
 Documentation/dev-tools/kasan.rst | 31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

Comments

Marco Elver March 12, 2021, 10:52 a.m. UTC | #1

On Thu, Mar 11, 2021 at 10:37PM +0100, Andrey Konovalov wrote:
[...]
> -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
>  ~~~~~~~~~~

While we're here, can we change this "By default" heading which seems
wrong -- the paragraph starts with "By default, ..." as well.

Perhaps "Default Behaviour"?

Andrey Konovalov March 12, 2021, 1:52 p.m. UTC | #2

On Fri, Mar 12, 2021 at 11:52 AM Marco Elver <[e-mail]> wrote:
>
> On Thu, Mar 11, 2021 at 10:37PM +0100, Andrey Konovalov wrote:
> [...]
> > -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
> >  ~~~~~~~~~~
>
> While we're here, can we change this "By default" heading which seems
> wrong -- the paragraph starts with "By default, ..." as well.
>
> Perhaps "Default Behaviour"?

Sounds good, thanks!

Patch

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