timer API transition for kernel 4.15

timer API transition for kernel 4.15

The timer API changes starting from kernel 4.15.0.

There's an interresting LWN article on this subject:

  https://lwn.net/Articles/735887/

Check these upstream commits for more details:

  commit 686fef928bba6be13cabe639f154af7d72b63120
  Author: Kees Cook <keescook@chromium.org>
  Date:   Thu Sep 28 06:38:17 2017 -0700

    timer: Prepare to change timer callback argument type

    Modern kernel callback systems pass the structure associated with a
    given callback to the callback function. The timer callback remains one
    of the legacy cases where an arbitrary unsigned long argument continues
    to be passed as the callback argument. This has several problems:

    - This bloats the timer_list structure with a normally redundant
      .data field.

    - No type checking is being performed, forcing callbacks to do
      explicit type casts of the unsigned long argument into the object
      that was passed, rather than using container_of(), as done in most
      of the other callback infrastructure.

    - Neighboring buffer overflows can overwrite both the .function and
      the .data field, providing attackers with a way to elevate from a buffer
      overflow into a simplistic ROP-like mechanism that allows calling
      arbitrary functions with a controlled first argument.

    - For future Control Flow Integrity work, this creates a unique function
      prototype for timer callbacks, instead of allowing them to continue to
      be clustered with other void functions that take a single unsigned long
      argument.

    This adds a new timer initialization API, which will ultimately replace
    the existing setup_timer(), setup_{deferrable,pinned,etc}_timer() family,
    named timer_setup() (to mirror hrtimer_setup(), making instances of its
    use much easier to grep for).

    In order to support the migration of existing timers into the new
    callback arguments, timer_setup() casts its arguments to the existing
    legacy types, and explicitly passes the timer pointer as the legacy
    data argument. Once all setup_*timer() callers have been replaced with
    timer_setup(), the casts can be removed, and the data argument can be
    dropped with the timer expiration code changed to just pass the timer
    to the callback directly.

:
    Modern kernel callback systems pass the structure associated with a
    given callback to the callback function. The timer callback remains one
    of the legacy cases where an arbitrary unsigned long argument continues
    to be passed as the callback argument. This has several problems:

    - This bloats the timer_list structure with a normally redundant
      .data field.

    - No type checking is being performed, forcing callbacks to do
      explicit type casts of the unsigned long argument into the object
      that was passed, rather than using container_of(), as done in most
      of the other callback infrastructure.

    - Neighboring buffer overflows can overwrite both the .function and
      the .data field, providing attackers with a way to elevate from a buffer
      overflow into a simplistic ROP-like mechanism that allows calling
      arbitrary functions with a controlled first argument.

    - For future Control Flow Integrity work, this creates a unique function
      prototype for timer callbacks, instead of allowing them to continue to
      be clustered with other void functions that take a single unsigned long
      argument.

    This adds a new timer initialization API, which will ultimately replace
    the existing setup_timer(), setup_{deferrable,pinned,etc}_timer() family,
    named timer_setup() (to mirror hrtimer_setup(), making instances of its
    use much easier to grep for).

    In order to support the migration of existing timers into the new
    callback arguments, timer_setup() casts its arguments to the existing
    legacy types, and explicitly passes the timer pointer as the legacy
    data argument. Once all setup_*timer() callers have been replaced with
    timer_setup(), the casts can be removed, and the data argument can be
    dropped with the timer expiration code changed to just pass the timer
    to the callback directly.

    Since the regular pattern of using container_of() during local variable
    declaration repeats the need for the variable type declaration
    to be included, this adds a helper modeled after other from_*()
    helpers that wrap container_of(), named from_timer(). This helper uses
    typeof(*variable), removing the type redundancy and minimizing the need
    for line wraps in forthcoming conversions from "unsigned data long" to
    "struct timer_list *" in the timer callbacks:

    -void callback(unsigned long data)
    +void callback(struct timer_list *t)
    {
    -   struct some_data_structure *local = (struct some_data_structure *)data;
    +   struct some_data_structure *local = from_timer(local, t, timer);

    Finally, in order to support the handful of timer users that perform
    open-coded assignments of the .function (and .data) fields, provide
    cast macros (TIMER_FUNC_TYPE and TIMER_DATA_TYPE) that can be used
    temporarily. Once conversion has been completed, these can be globally
    trivially removed.

    ...

  commit e99e88a9d2b067465adaa9c111ada99a041bef9a
  Author: Kees Cook <keescook@chromium.org>
  Date:   Mon Oct 16 14:43:17 2017 -0700

    treewide: setup_timer() -> timer_setup()

    This converts all remaining cases of the old setup_timer() API into using
    timer_setup(), where the callback argument is the structure already
    holding the struct timer_list. These should have no behavioral changes,
    since they just change which pointer is passed into the callback with
    the same available pointers after conversion. It handles the following
    examples, in addition to some other variations.

    ...

  commit 185981d54a60ae90942c6ba9006b250f3348cef2
  Author: Kees Cook <keescook@chromium.org>
  Date:   Wed Oct 4 16:26:58 2017 -0700

    timer: Remove init_timer_pinned() in favor of timer_setup()

    This refactors the only users of init_timer_pinned() to use
    the new timer_setup() and from_timer(). Drops the definition of
    init_timer_pinned().

    ...

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>