Message-ID: <YEuD1Ghn+5bf0TJO@elver.google.com>
Date:   Fri, 12 Mar 2021 16:08:04 +0100
From:   Marco Elver <elver@...gle.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 04/11] kasan: docs: update error reports section

On Fri, Mar 12, 2021 at 03:24PM +0100, Andrey Konovalov wrote:
> Update the "Error reports" section in KASAN documentation:
> 
> - Mention that bug titles are best-effort.
> - Move and reword the part about auxiliary stacks from
>   "Implementation details".
> - Punctuation, readability, and other minor clean-ups.
> 
> Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>

Reviewed-by: Marco Elver <elver@...gle.com>

> ---
>  Documentation/dev-tools/kasan.rst | 46 +++++++++++++++++--------------
>  1 file changed, 26 insertions(+), 20 deletions(-)
> 
> diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst
> index 46f4e9680805..cd12c890b888 100644
> --- a/Documentation/dev-tools/kasan.rst
> +++ b/Documentation/dev-tools/kasan.rst
> @@ -60,7 +60,7 @@ physical pages, enable ``CONFIG_PAGE_OWNER`` and boot with ``page_owner=on``.
>  Error reports
>  ~~~~~~~~~~~~~
>  
> -A typical out-of-bounds access generic KASAN report looks like this::
> +A typical KASAN report looks like this::
>  
>      ==================================================================
>      BUG: KASAN: slab-out-of-bounds in kmalloc_oob_right+0xa8/0xbc [test_kasan]
> @@ -133,33 +133,43 @@ A typical out-of-bounds access generic KASAN report looks like this::
>       ffff8801f44ec400: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>      ==================================================================
>  
> -The header of the report provides a short summary of what kind of bug happened
> -and what kind of access caused it. It's followed by a stack trace of the bad
> -access, a stack trace of where the accessed memory was allocated (in case bad
> -access happens on a slab object), and a stack trace of where the object was
> -freed (in case of a use-after-free bug report). Next comes a description of
> -the accessed slab object and information about the accessed memory page.
> +The report header summarizes what kind of bug happened and what kind of access
> +caused it. It is followed by a stack trace of the bad access, a stack trace of
> +where the accessed memory was allocated (in case a slab object was accessed),
> +and a stack trace of where the object was freed (in case of a use-after-free
> +bug report). Next comes a description of the accessed slab object and the
> +information about the accessed memory page.
>  
> -In the last section the report shows memory state around the accessed address.
> -Internally KASAN tracks memory state separately for each memory granule, which
> +In the end, the report shows the memory state around the accessed address.
> +Internally, KASAN tracks memory state separately for each memory granule, which
>  is either 8 or 16 aligned bytes depending on KASAN mode. Each number in the
>  memory state section of the report shows the state of one of the memory
>  granules that surround the accessed address.
>  
> -For generic KASAN the size of each memory granule is 8. The state of each
> +For generic KASAN, the size of each memory granule is 8. The state of each
>  granule is encoded in one shadow byte. Those 8 bytes can be accessible,
> -partially accessible, freed or be a part of a redzone. KASAN uses the following
> -encoding for each shadow byte: 0 means that all 8 bytes of the corresponding
> +partially accessible, freed, or be a part of a redzone. KASAN uses the following
> +encoding for each shadow byte: 00 means that all 8 bytes of the corresponding
>  memory region are accessible; number N (1 <= N <= 7) means that the first N
>  bytes are accessible, and other (8 - N) bytes are not; any negative value
>  indicates that the entire 8-byte word is inaccessible. KASAN uses different
>  negative values to distinguish between different kinds of inaccessible memory
>  like redzones or freed memory (see mm/kasan/kasan.h).
>  
> -In the report above the arrows point to the shadow byte 03, which means that
> -the accessed address is partially accessible. For tag-based KASAN modes this
> -last report section shows the memory tags around the accessed address
> -(see the `Implementation details`_ section).
> +In the report above, the arrow points to the shadow byte ``03``, which means
> +that the accessed address is partially accessible.
> +
> +For tag-based KASAN modes, this last report section shows the memory tags around
> +the accessed address (see the `Implementation details`_ section).
> +
> +Note that KASAN bug titles (like ``slab-out-of-bounds`` or ``use-after-free``)
> +are best-effort: KASAN prints the most probable bug type based on the limited
> +information it has. The actual type of the bug might be different.
> +
> +Generic KASAN also reports up to two auxiliary call stack traces. These stack
> +traces point to places in code that interacted with the object but that are not
> +directly present in the bad access stack trace. Currently, this includes
> +call_rcu() and workqueue queuing.
>  
>  Boot parameters
>  ~~~~~~~~~~~~~~~
> @@ -214,10 +224,6 @@ 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.
>  
> -Generic KASAN also reports the last 2 call stacks to creation of work that
> -potentially has access to an object. Call stacks for the following are shown:
> -call_rcu() and workqueue queuing.
> -
>  Generic KASAN is the only mode that delays the reuse of freed object via
>  quarantine (see mm/kasan/quarantine.c for implementation).
>  
> -- 
> 2.31.0.rc2.261.g7f71774620-goog
> 

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