[RFC PATCH 13/13] Documentation/dev-tools: document KWatch

[Jinchao Wang <wangjinchao600@gmail.com>]

Describe what KWatch is for, how it compares with KASAN and KFENCE,
the debugfs configuration interface, the watch expression syntax,
how to read hits from the trace buffer (including after a crash),
and the current limitations.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 Documentation/dev-tools/index.rst  |   1 +
 Documentation/dev-tools/kwatch.rst | 193 +++++++++++++++++++++++++++++
 2 files changed, 194 insertions(+)
 create mode 100644 Documentation/dev-tools/kwatch.rst

diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 59cbb77b33ff..f4c748da63db 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -30,6 +30,7 @@ Documentation/process/debugging/index.rst
    ubsan
    kmemleak
    kcsan
+   kwatch
    lkmm/index
    kfence
    kselftest
diff --git a/Documentation/dev-tools/kwatch.rst b/Documentation/dev-tools/kwatch.rst
new file mode 100644
index 000000000000..8ead0beb06b6
--- /dev/null
+++ b/Documentation/dev-tools/kwatch.rst
@@ -0,0 +1,193 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================================
+KWatch - Kernel Memory Watchpoint Tool
+======================================
+
+Overview
+========
+
+KWatch is a runtime-configurable debugging tool for locating kernel memory
+corruption. It arms hardware breakpoints (watchpoints) on a target address
+while a chosen function is executing, and reports the exact instruction that
+touches the watched memory, together with a stack trace, through a
+tracepoint.
+
+Unlike shadow-memory sanitizers, KWatch does not detect invalid accesses in
+general; it answers a narrower but common question during corruption hunts:
+"who writes to this address?". This includes in-bounds logical overwrites
+that KASAN cannot see, because the rogue writer modifies valid memory
+through a valid pointer, just at the wrong time or with the wrong data.
+
+Comparison with other tools:
+
+* KASAN detects out-of-bounds and use-after-free accesses, but reports the
+  symptom (the invalid access), not the writer that corrupted the data
+  earlier. It requires a rebuild and has significant CPU and memory
+  overhead, and its redzones perturb memory layout, which can hide
+  timing-sensitive bugs.
+* KFENCE is a low-overhead sampling detector for slab objects; it cannot be
+  pointed at one specific address.
+* Hardware breakpoints via kgdb or perf can watch an address, but only a
+  fixed one, system-wide, for the whole run. KWatch resolves the address
+  dynamically at function entry (for example "argument 2 of this function,
+  plus offset 8, dereferenced once") and disarms it again at function exit,
+  so short-lived and per-invocation objects can be watched too.
+
+KWatch has near-zero overhead while armed: the watched function pays for
+one kprobe/kretprobe pair plus programming of the debug registers; the rest
+of the system runs at full speed.
+
+Requirements
+============
+
+* ``CONFIG_KWATCH=y`` or ``m``. The Kconfig symbol depends on
+  ``CONFIG_PERF_EVENTS``, ``CONFIG_DEBUG_FS`` and an architecture that
+  provides ``HAVE_REINSTALL_HW_BREAKPOINT`` (currently x86 only).
+* Resolving symbol names in watch expressions requires ``CONFIG_KWATCH=y``
+  (built-in); a module can only watch absolute hexadecimal addresses.
+
+Usage
+=====
+
+KWatch is configured through a single debugfs file::
+
+    /sys/kernel/debug/kwatch/config
+
+Writing a configuration string starts a watch session (stopping any previous
+one); reading the file shows the active configuration and hit-rejection
+counters. The configuration is a whitespace-separated list of ``key=value``
+tokens:
+
+=================== ==========================================================
+Key                 Meaning
+=================== ==========================================================
+``func_name``       Function whose execution opens the watch window.
+``func_offset``     Instruction offset inside ``func_name`` at which the
+                    watchpoint is armed (default 0 = function entry).
+``watch_expr``      Expression describing the address to watch (see below).
+``watch_len``       Watched length in bytes: 1, 2, 4 or 8 (default 8).
+``access_type``     0 = write (default), 1 = read, 2 = read/write,
+                    3 = execute.
+``depth``           Recursion depth at which the window opens (default 0).
+``max_watch``       Number of hardware watchpoints to preallocate
+                    (default 4).
+``max_concurrency`` Maximum number of tasks concurrently inside the watch
+                    window (default 256).
+``duration``        For global watches: seconds until automatic stop.
+=================== ==========================================================
+
+Watch expressions
+-----------------
+
+The address to watch is computed at function entry from::
+
+    watch_expr={base}[+-offset][->[+-]offset]...
+
+* ``base`` is one of:
+
+  - ``arg1`` ... ``arg6``: a function argument (register calling
+    convention),
+  - ``stack``: the kernel stack pointer at the probe point,
+  - an absolute hexadecimal address, e.g. ``0xffffffff81234567``,
+  - a global symbol name (built-in KWatch only).
+
+* ``+offset`` / ``-offset`` adjusts the current address.
+* ``->offset`` loads the pointer stored at the current address (via
+  ``get_kernel_nofault()``) and then applies the offset. Up to four chain
+  elements are supported; offsets must be explicit (``->`` alone is
+  rejected).
+
+Given::
+
+    struct some_struct {
+        struct some_struct *ptr;    /* offset 0 */
+        int num;                    /* offset 8 */
+    };
+
+    void target_function(struct some_struct *arg1);
+
+typical expressions are:
+
+=========================== ==============================================
+Expression                  Watches
+=========================== ==============================================
+``watch_expr=arg1``         ``&arg1->ptr`` (the pointer field itself)
+``watch_expr=arg1+8``       ``&arg1->num``
+``watch_expr=arg1->0``      ``&arg1->ptr->ptr`` (one dereference)
+``watch_expr=arg1->8``      ``&arg1->ptr->num``
+``watch_expr=0xffff...+8``  absolute address plus 8
+=========================== ==============================================
+
+Example: catch whoever overwrites ``arg1->num`` of a function while that
+function runs::
+
+    echo "func_name=target_function watch_expr=arg1+8 watch_len=4" \
+        > /sys/kernel/debug/kwatch/config
+
+Watching global variables
+-------------------------
+
+A global variable has no natural function window. When ``duration`` is
+given without ``func_name``, KWatch starts an internal anchor kernel thread
+that sleeps inside a dummy function, and uses that function as the window::
+
+    echo "watch_expr=jiffies_wobble duration=60 watch_len=8" \
+        > /sys/kernel/debug/kwatch/config
+
+The session tears itself down when the duration expires.
+
+Reading hits
+------------
+
+Hits are emitted as the ``kwatch:kwatch_hit`` tracepoint, which is safe in
+NMI-like contexts where printk is not. Each event carries the timestamp,
+the instruction pointer, the watched address and a short stack trace::
+
+    echo 1 > /sys/kernel/debug/tracing/events/kwatch/kwatch_hit/enable
+    cat /sys/kernel/debug/tracing/trace_pipe
+
+If the corruption crashes the machine, the ring buffer can still be
+recovered:
+
+* ``echo 1 > /proc/sys/kernel/ftrace_dump_on_oops`` (or the
+  ``ftrace_dump_on_oops`` boot parameter) dumps the buffer to the console
+  on an oops.
+* With kdump, the buffer is present in the vmcore and can be read with
+  ``crash> trace``.
+* ``CONFIG_PSTORE_FTRACE`` persists it across reboots on supported
+  platforms.
+
+Limitations
+===========
+
+* Functions that run in a genuine NMI(-like) context are rejected at
+  function entry; rejected invocations never open a watch window and are
+  counted in the ``nmi_rejected`` field of the config file. Watching
+  functions reachable from NMI handlers is out of scope.
+* The number of concurrent watchpoints is bounded by the CPU's debug
+  registers (typically 4).
+* If the target address cannot be resolved at arming time (for example a
+  ``get_kernel_nofault()`` failure on a swapped or unmapped page), the
+  watchpoint is not armed for that invocation.
+* Offsets in watch expressions are static; dynamic indexing such as
+  ``arg1->ptr[arg2]`` is not supported.
+* arm64 is not yet supported: stepping over a hit that has a custom
+  overflow handler needs a generic mechanism in the arch code, which is
+  planned as a follow-up series.
+
+Implementation notes
+====================
+
+The implementation lives in ``mm/kwatch/`` and is split into a control
+plane (``core.c``, the debugfs interface), an execution plane (``probe.c``
+and ``deref.c``: kprobe/kretprobe window management and address
+resolution), and a resource plane (``hwbp.c`` and ``task_ctx.c``).
+
+Hardware watchpoints are preallocated as perf events on every CPU and
+re-pointed at hit time with ``modify_wide_hw_breakpoint_local()``, a new
+hw_breakpoint API that updates the breakpoint on the local CPU without
+releasing its slot; other CPUs are updated by asynchronous IPIs. Per-task
+window state is kept in a fixed-size, lockless open-addressing array
+claimed with ``cmpxchg()``, so the hit path performs no allocation and
+takes no locks, which keeps it safe in atomic and NMI-like contexts.
-- 
2.53.0