[PATCH v2 2/2] docs: fault-injection: Add requirements of error injectable functions

["Masami Hiramatsu (Google)" <mhiramat@kernel.org>]

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Add a section about the requirements of the error injectable functions
and the type of errors.
Since this section must be read before using ALLOW_ERROR_INJECTION()
macro, that section is referred from the comment of the macro too.

Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Link: https://lore.kernel.org/all/20221211115218.2e6e289bb85f8cf53c11aa97@kernel.org/T/#u
---
 Changes in v2:
  - Fix typos and misses according to Randy's comment.
---
 Documentation/fault-injection/fault-injection.rst |   65 +++++++++++++++++++++
 include/asm-generic/error-injection.h             |    6 +-
 2 files changed, 69 insertions(+), 2 deletions(-)

diff --git a/Documentation/fault-injection/fault-injection.rst b/Documentation/fault-injection/fault-injection.rst
index 17779a2772e5..76bc9c857761 100644
--- a/Documentation/fault-injection/fault-injection.rst
+++ b/Documentation/fault-injection/fault-injection.rst
@@ -233,6 +233,71 @@ proc entries
 	This feature is intended for systematic testing of faults in a single
 	system call. See an example below.
 
+
+Error Injectable Functions
+--------------------------
+
+This part is for the kenrel developers considering adding a function
+using the ALLOW_ERROR_INJECTION() macro.
+
+Requirements for the Error Injectable Functions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Since the function-level error injection forcibly changes the code path
+and returns an error even if the input and conditions are proper, this can
+cause unexpected kernel crash if you allow error injection on the function
+which is NOT error injectable. Thus, you (and reviewers) must ensure;
+
+- The function returns an error code if it fails, and the callers must check
+  it correctly (need to recover from it).
+
+- The function does not execute any code which can change any state before
+  the first error return. The state includes global or local, or input
+  variable. For example, clear output address storage (e.g. `*ret = NULL`),
+  increment/decrement counter, set a flag, preempt/irq disable or get
+  a lock (if those are recovered before returning error, that will be OK.)
+
+The first requirement is important, and it will result in that the release
+(free objects) functions are usually harder to inject errors than allocate
+functions. If errors of such release functions are not correctly handled
+it will cause a memory leak easily (the caller will confuse that the object
+has been released or corrupted.)
+
+The second one is for the caller which expects the function should always
+do something. Thus if the function error injection skips all of the
+function, the expectation is betrayed and causes an unexpected error.
+
+Type of the Error Injectable Functions
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Each error injectable function will have the error type specified by the
+ALLOW_ERROR_INJECTION() macro. You have to choose it carefully if you add
+a new error injectable function. If the wrong error type is chosen, the
+kernel may crash because it may not be able to handle the error.
+There are 4 types of errors defined in include/asm-generic/error-injection.h
+
+EI_ETYPE_NULL
+  This function will return `NULL` if it fails. e.g. return an allocated
+  object address.
+
+EI_ETYPE_ERRNO
+  This function will return an `-errno` error code if it fails. e.g. return
+  -EINVAL if the input is wrong. This will include the functions which will
+  return an address which encodes `-errno` by ERR_PTR() macro.
+
+EI_ETYPE_ERRNO_NULL
+  This function will return an `-errno` or `NULL` if it fails. If the caller
+  of this function checks the return value with IS_ERR_OR_NULL() macro, this
+  type will be appropriate.
+
+EI_ETYPE_TRUE
+  This function will return `true` (non-zero positive value) if it fails.
+
+If you specify a wrong type, for example, EI_TYPE_ERRNO for the function
+which returns an allocated object, it may cause a problem because the returned
+value is not an object address and the caller can not access to the address.
+
+
 How to add new fault injection capability
 -----------------------------------------
 
diff --git a/include/asm-generic/error-injection.h b/include/asm-generic/error-injection.h
index c0b9d3217ed9..18324d7aa56d 100644
--- a/include/asm-generic/error-injection.h
+++ b/include/asm-generic/error-injection.h
@@ -19,8 +19,10 @@ struct pt_regs;
 
 #ifdef CONFIG_FUNCTION_ERROR_INJECTION
 /*
- * Whitelist generating macro. Specify functions which can be
- * error-injectable using this macro.
+ * Whitelist generating macro. Specify functions which can be error-injectable
+ * using this macro. If you are unsure what is required for the error-injectable
+ * functions, please read Documentation/fault-injection/fault-injection.rst
+ * 'Error Injectable Functions' section.
  */
 #define ALLOW_ERROR_INJECTION(fname, _etype)				\
 static struct error_injection_entry __used				\