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

[Randy Dunlap]

Hi--

On 12/11/22 18:46, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/20221211115218.2e6e289bb85f8cf53c11aa97@xxxxxxxxxx/T/#u
> ---
>  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..da6c5796b1f8 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 to add a function to

                        kernel developers considering adding a function

> +ALLOW_ERROR_INJECTION() macro.

   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`),
> +  increments/decrements counter, set a flag, preempt/irq disable or get

     increment/decrement a counter,

> +  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
> +does something. Thus if the function error injection skips whole of the

   do something.                                        skips all of the

> +function, the expectation is betrayed and causes an unexpected error.
> +
> +Type of the Error Injectable Functions
> +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> +
> +Each error injectable functions will have the error type specified by the

                         function

> +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 allocateed

                                                                  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 specifies a wrong type, for example, EI_TYPE_ERRNO for the function

          specify

> +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..b05253f68eaa 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 unsure what is required for the error-injectable

                        If you are unsure ...

> + * 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				\
> 

-- 
~Randy