Re: [PATCH] bug: Switch data corruption check to __must_check

[Paul E. McKenney]

On Tue, Feb 07, 2017 at 12:57:33PM -0800, Kees Cook wrote:
> On Tue, Feb 7, 2017 at 12:39 PM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > On Mon, Feb 06, 2017 at 12:45:47PM -0800, Kees Cook wrote:
> >> The CHECK_DATA_CORRUPTION() macro was designed to have callers do
> >> something meaningful/protective on failure. However, using "return false"
> >> in the macro too strictly limits the design patterns of callers. Instead,
> >> let callers handle the logic test directly, but make sure that the result
> >> IS checked by forcing __must_check (which appears to not be able to be
> >> used directly on macro expressions).
> >>
> >> Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
> >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> >> ---
> >>  include/linux/bug.h | 12 +++++++-----
> >>  lib/list_debug.c    | 45 ++++++++++++++++++++++++---------------------
> >>  2 files changed, 31 insertions(+), 26 deletions(-)
> >>
> >> diff --git a/include/linux/bug.h b/include/linux/bug.h
> >> index baff2e8fc8a8..5828489309bb 100644
> >> --- a/include/linux/bug.h
> >> +++ b/include/linux/bug.h
> >> @@ -124,18 +124,20 @@ static inline enum bug_trap_type report_bug(unsigned long bug_addr,
> >>
> >>  /*
> >>   * Since detected data corruption should stop operation on the affected
> >> - * structures, this returns false if the corruption condition is found.
> >> + * structures. Return value must be checked and sanely acted on by caller.
> >>   */
> >> +static inline __must_check bool check_data_corruption(bool v) { return v; }
> >>  #define CHECK_DATA_CORRUPTION(condition, fmt, ...)                    \
> >> -     do {                                                             \
> >> -             if (unlikely(condition)) {                               \
> >> +     check_data_corruption(({                                         \
> >
> > The definition of check_data_corruption() is in some other patch?  I don't
> > see it in current mainline.  I am not seeing what it might be doing.
> 
> It's immediately before the #define line above. It's nothing more than
> an inline argument pass-through, but since it's a _function_ I can
> attach __must_check to it, which I can't do for a conditional
> expression macro. And I gave it the meaningful name so when someone
> fails to check CHECK_DATA_CORRUPTION, they'll get a gcc warning about
> "check_data_corruption" which will lead them here.

Ah, I see it now.  Color me blind!

							Thanx, Paul

> >> +             bool corruption = unlikely(condition);                   \
> >
> > So corruption = unlikely(condition)?  Sounds a bit optimistic to me!  ;-)
> 
> It's true though! :) Nearly all calls to CHECK_DATA_CORRUPTION()
> should end up with a false condition.
> 
> >
> >> +             if (corruption) {                                        \
> >>                       if (IS_ENABLED(CONFIG_BUG_ON_DATA_CORRUPTION)) { \
> >>                               pr_err(fmt, ##__VA_ARGS__);              \
> >>                               BUG();                                   \
> >>                       } else                                           \
> >>                               WARN(1, fmt, ##__VA_ARGS__);             \
> >> -                     return false;                                    \
> >>               }                                                        \
> >> -     } while (0)
> >> +             corruption;                                              \
> >> +     }))
> >>
> >>  #endif       /* _LINUX_BUG_H */
> >> diff --git a/lib/list_debug.c b/lib/list_debug.c
> >> index 7f7bfa55eb6d..a34db8d27667 100644
> >> --- a/lib/list_debug.c
> >> +++ b/lib/list_debug.c
> >> @@ -20,15 +20,16 @@
> >>  bool __list_add_valid(struct list_head *new, struct list_head *prev,
> >>                     struct list_head *next)
> >>  {
> >> -     CHECK_DATA_CORRUPTION(next->prev != prev,
> >> -             "list_add corruption. next->prev should be prev (%p), but was %p. (next=%p).\n",
> >> -             prev, next->prev, next);
> >> -     CHECK_DATA_CORRUPTION(prev->next != next,
> >> -             "list_add corruption. prev->next should be next (%p), but was %p. (prev=%p).\n",
> >> -             next, prev->next, prev);
> >> -     CHECK_DATA_CORRUPTION(new == prev || new == next,
> >> -             "list_add double add: new=%p, prev=%p, next=%p.\n",
> >> -             new, prev, next);
> >> +     if (CHECK_DATA_CORRUPTION(next->prev != prev,
> >> +                     "list_add corruption. next->prev should be prev (%p), but was %p. (next=%p).\n",
> >> +                     prev, next->prev, next) ||
> >> +         CHECK_DATA_CORRUPTION(prev->next != next,
> >> +                     "list_add corruption. prev->next should be next (%p), but was %p. (prev=%p).\n",
> >> +                     next, prev->next, prev) ||
> >> +         CHECK_DATA_CORRUPTION(new == prev || new == next,
> >> +                     "list_add double add: new=%p, prev=%p, next=%p.\n",
> >> +                     new, prev, next))
> >> +             return false;
> >
> > That -is- one ornate "if" condition, isn't it?
> 
> It is, yes. :)
> 
> > Still it is nice to avoid the magic return from out of the middle of the
> > C-preprocessor macro.
> 
> Agreed. I had fun with indenting to make it passably readable. :P
> 
> -Kees
> 
> -- 
> Kees Cook
> Pixel Security
>