[PATCH v3 19/19] docs: add KStackWatch document

Jinchao Wang
2025-09-10T05:34:09Z

Add a new documentation file for KStackWatch, explaining its
purpose, motivation, key features, configuration format, module parameters,
implementation notes, limitations, and testing instructions.

Update MAINTAINERS to include Jinchao Wang as the maintainer for associated
files.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
Documentation/dev-tools/kstackwatch.rst | 94 +++++++++++++++++++++++++
MAINTAINERS | 7 ++
2 files changed, 101 insertions(+)
create mode 100644 Documentation/dev-tools/kstackwatch.rst

diff --git a/Documentation/dev-tools/kstackwatch.rst b/Documentation/dev-tools/kstackwatch.rst
new file mode 100644
index 000000000000..f741de08ca56
--- /dev/null
+++ b/Documentation/dev-tools/kstackwatch.rst
@@ -0,0 +1,94 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+====================================
+KStackWatch: Kernel Stack Watch
+====================================
+
+Overview
+========
+KStackWatch is a lightweight debugging tool designed to detect
+kernel stack corruption in real time. It helps developers capture the
+moment corruption occurs, rather than only observing a later crash.
+
+Motivation
+==========
+Stack corruption may originate in one function but manifest much later
+with no direct call trace linking the two. This makes such issues
+extremely difficult to diagnose. KStackWatch addresses this by combining
+hardware breakpoints with kprobe and fprobe instrumentation, monitoring
+stack canaries or local variables at the point of corruption.
+
+Key Features
+============
+- Lightweight overhead:
+ Minimal runtime cost, preserving bug reproducibility.
+- Real-time detection:
+ Detect stack corruption immediately.
+- Flexible configuration:
+ Control via a procfs interface.
+- Depth filtering:
+ Optional recursion depth tracking per task.
+
+Configuration
+=============
+The control file is created at::
+
+ /proc/kstackwatch
+
+To configure, write a string in the following format::
+
+ function+ip_offset[+depth] [local_var_offset:local_var_len]
+ - function : name of the target function
+ - ip_offset : instruction pointer offset within the function
+ - depth : recursion depth to watch, starting from 0
+ - local_var_offset : offset from the stack pointer at function+ip_offset
+ - local_var_len : length of the local variable(1,2,4,8)
+
+Fields
+------
+- ``function``:
+ Name of the target function to watch.
+- ``ip_offset``:
+ Instruction pointer offset within the function.
+- ``depth`` (optional):
+ Maximum recursion depth for the watch.
+- ``local_var_offset:local_var_len`` (optional):
+ A region of a local variable to monitor, relative to the stack pointer.
+ If not given, KStackWatch monitors the stack canary by default.
+
+Examples
+--------
+1. Watch the canary at the entry of ``canary_test_write``::
+
+ echo 'canary_test_write+0x12' > /proc/kstackwatch
+
+2. Watch a local variable of 8 bytes at offset 0 in
+ ``silent_corruption_victim``::
+
+ echo 'silent_corruption_victim+0x7f 0:8' > /proc/kstackwatch
+
+Module Parameters
+=================
+``panic_on_catch`` (bool)
+ - If true, trigger a kernel panic immediately on detecting stack
+ corruption.
+ - Default is false (log a message only).
+
+Implementation Notes
+====================
+- Hardware breakpoints are preallocated at watch start.
+- Function exit is monitored using ``fprobe``.
+- Per-task depth tracking is used to handle recursion across scheduling.
+- The procfs interface allows dynamic reconfiguration at runtime.
+- Active state is cleared before applying new settings.
+
+Limitations
+===========
+- Only one active watch can be configured at a time (singleton).
+- Local variable offset and size must be known in advance.
+
+Testing
+=======
+KStackWatch includes a companion test module (`kstackwatch_test`) and
+a helper script (`kstackwatch_test.sh`) to exercise different stack
+corruption scenarios:
diff --git a/MAINTAINERS b/MAINTAINERS
index cd7ff55b5d32..076512afddcc 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -13355,6 +13355,13 @@ T: git git://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git
F: Documentation/dev-tools/kselftest*
F: tools/testing/selftests/

+KERNEL STACK WATCH
+M: Jinchao Wang <wangjinchao600@gmail.com>
+S: Maintained
+F: Documentation/dev-tools/kstackwatch.rst
+F: mm/kstackwatch/
+F: tools/kstackwatch/
+
KERNEL SMB3 SERVER (KSMBD)
M: Namjae Jeon <linkinjeon@kernel.org>
M: Namjae Jeon <linkinjeon@samba.org>
--
2.43.0