[PATCH] docs: lkdtm: Modernize and improve details

[Kees Cook]

The details on using LKDTM were overly obscure. Modernize the details
and expand examples to better illustrate how to use the interfaces.
Additionally add missing SPDX header.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 .../fault-injection/provoke-crashes.rst       | 56 +++++++++++--------
 1 file changed, 33 insertions(+), 23 deletions(-)

diff --git a/Documentation/fault-injection/provoke-crashes.rst b/Documentation/fault-injection/provoke-crashes.rst
index 9279a3e12278..93775bd4e6c8 100644
--- a/Documentation/fault-injection/provoke-crashes.rst
+++ b/Documentation/fault-injection/provoke-crashes.rst
@@ -1,16 +1,19 @@
-===============
-Provoke crashes
-===============
+.. SPDX-License-Identifier: GPL-2.0
 
-The lkdtm module provides an interface to crash or injure the kernel at
-predefined crashpoints to evaluate the reliability of crash dumps obtained
-using different dumping solutions. The module uses KPROBEs to instrument
-crashing points, but can also crash the kernel directly without KRPOBE
-support.
+============================================================
+Provoking crashes with Linux Kernel Dump Test Module (LKDTM)
+============================================================
 
+The lkdtm module provides an interface to disrupt (and usually crash)
+the kernel at predefined code locations to evaluate the reliability of
+the kernel's exception handling and to test crash dumps obtained using
+different dumping solutions. The module uses KPROBEs to instrument the
+trigger location, but can also trigger the kernel directly without KPROBE
+support via debugfs.
 
-You can provide the way either through module arguments when inserting
-the module, or through a debugfs interface.
+You can select the location of the trigger ("crash point name") and the
+type of action ("crash point type") either through module arguments when
+inserting the module, or through the debugfs interface.
 
 Usage::
 
@@ -18,31 +21,38 @@ Usage::
 			[cpoint_count={>0}]
 
 recur_count
-	Recursion level for the stack overflow test. Default is 10.
+	Recursion level for the stack overflow test. By default this is
+	dynamically calculated based on kernel configuration, with the
+	goal of being just large enough to exhaust the kernel stack. The
+	value can be seen at `/sys/module/lkdtm/parameters/recur_count`.
 
 cpoint_name
-	Crash point where the kernel is to be crashed. It can be
+	Where in the kernel to trigger the action. It can be
 	one of INT_HARDWARE_ENTRY, INT_HW_IRQ_EN, INT_TASKLET_ENTRY,
 	FS_DEVRW, MEM_SWAPOUT, TIMERADD, SCSI_DISPATCH_CMD,
-	IDE_CORE_CP, DIRECT
+	IDE_CORE_CP, or DIRECT
 
 cpoint_type
 	Indicates the action to be taken on hitting the crash point.
-	It can be one of PANIC, BUG, EXCEPTION, LOOP, OVERFLOW,
-	CORRUPT_STACK, UNALIGNED_LOAD_STORE_WRITE, OVERWRITE_ALLOCATION,
-	WRITE_AFTER_FREE,
+	These are numerous, and best queried directly from debugfs. Some
+	of the common ones are PANIC, BUG, EXCEPTION, LOOP, and OVERFLOW.
+	See the contents of `/sys/kernel/debug/provoke-crash/DIRECT` for
+	a complete list.
 
 cpoint_count
 	Indicates the number of times the crash point is to be hit
-	to trigger an action. The default is 10.
+	before triggering the action. The default is 10 (except for
+	DIRECT, which always fires immediately).
 
 You can also induce failures by mounting debugfs and writing the type to
-<mountpoint>/provoke-crash/<crashpoint>. E.g.::
+<debugfs>/provoke-crash/<crashpoint>. E.g.::
 
-  mount -t debugfs debugfs /mnt
-  echo EXCEPTION > /mnt/provoke-crash/INT_HARDWARE_ENTRY
+  mount -t debugfs debugfs /sys/kernel/debug
+  echo EXCEPTION > /sys/kernel/debug/provoke-crash/INT_HARDWARE_ENTRY
 
+The special file `DIRECT` will induce the action directly without KPROBE
+instrumentation. This mode is the only one available when the module is
+built for a kernel without KPROBEs support::
 
-A special file is `DIRECT` which will induce the crash directly without
-KPROBE instrumentation. This mode is the only one available when the module
-is built on a kernel without KPROBEs support.
+  # Instead of having a BUG kill your shell, have it kill "cat":
+  cat <(echo WRITE_RO) >/sys/kernel/debug/provoke-crash/DIRECT
-- 
2.25.1