Re: [RFC PATCH] rcu: rcupdate.h: Add missing parentheses around macro pointer dereference

[Paul E. McKenney]

On Wed, May 03, 2023 at 06:06:40PM -0400, Steven Rostedt wrote:
> On Wed,  3 May 2023 16:32:36 -0400
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> 
> > linux/rcupdate.h macros use the *p parameter without parentheses, e.g.:
> > 
> >   typeof(*p)
> > 
> > rather than
> > 
> >   typeof(*(p))
> > 
> > The following test-case shows how it can generate confusion due to C
> > operator precedence being reversed compared to the expectations:
> > 
> >     #define m(p) \
> >     do { \
> >             __typeof__(*p) v = 0; \
> >     } while (0)
> > 
> >     void fct(unsigned long long *p1)
> >     {
> >             m(p1 + 1);      /* works */
> >             m(1 + p1);      /* broken */
> >     }
> > 
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > Cc: Zqiang <qiang1.zhang@xxxxxxxxx>
> > ---
> >  include/linux/rcupdate.h | 18 +++++++++---------
> >  1 file changed, 9 insertions(+), 9 deletions(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index dcd2cf1e8326..1565012fa47f 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -430,16 +430,16 @@ static inline void rcu_preempt_sleep_check(void) { }
> >  
> >  #ifdef __CHECKER__
> >  #define rcu_check_sparse(p, space) \
> > -	((void)(((typeof(*p) space *)p) == p))
> > +	((void)(((typeof(*(p)) space *)p) == p))
> 
> Hmm, should we have that be:
> 	((void)(((typeof(*(p)) space *)(p)) == (p)))
> 
> In case of the 1 + p1, which would end up as:
> 
> 	((void)(((typeof(*(1 + p1)) __rcu *)1 + p1 == 1 + p1;
> 
> I don't know how that __rcu get's passed around via the + statement there,
> so it may be fine. May not even make sense to have that. But I like to
> error on more parenthesis. ;-)
> 
> The rest looks fine.
> 
> Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

Thank you all!  I applied Steve's suggested change with attribution
as shown below.  Please let me know if I messed anything up.

							Thanx, Paul

------------------------------------------------------------------------

commit d3d734216c88fb7c13205dc62178ff5011da415b
Author: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Date:   Wed May 3 16:32:36 2023 -0400

    rcu: Add missing parentheses around rcu_dereference() "p" parameter
    
    linux/rcupdate.h macros use the *p parameter without parentheses, e.g.:
    
      typeof(*p)
    
    rather than
    
      typeof(*(p))
    
    The following test-case shows how it can generate confusion due to C
    operator precedence being reversed compared to the expectations:
    
        #define m(p) \
        do { \
                __typeof__(*p) v = 0; \
        } while (0)
    
        void fct(unsigned long long *p1)
        {
                m(p1 + 1);      /* works */
                m(1 + p1);      /* broken */
        }
    
    [ paulmck: Apply Steve Rostedt additional () feedback. ]
    
    Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
    Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
    Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
    Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
    Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
    Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
    Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
    Cc: Zqiang <qiang1.zhang@xxxxxxxxx>
    Reviewed-by: Boqun Feng <boqun.feng@xxxxxxxxx>
    Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
    Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index ddd42efc6224..cb938a89a923 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -405,16 +405,16 @@ static inline void rcu_preempt_sleep_check(void) { }
 
 #ifdef __CHECKER__
 #define rcu_check_sparse(p, space) \
-	((void)(((typeof(*p) space *)p) == p))
+	((void)(((typeof(*(p)) space *)(p)) == (p)))
 #else /* #ifdef __CHECKER__ */
 #define rcu_check_sparse(p, space)
 #endif /* #else #ifdef __CHECKER__ */
 
 #define __unrcu_pointer(p, local)					\
 ({									\
-	typeof(*p) *local = (typeof(*p) *__force)(p);			\
+	typeof(*(p)) *local = (typeof(*(p)) *__force)(p);		\
 	rcu_check_sparse(p, __rcu);					\
-	((typeof(*p) __force __kernel *)(local)); 			\
+	((typeof(*(p)) __force __kernel *)(local));			\
 })
 /**
  * unrcu_pointer - mark a pointer as not being RCU protected
@@ -427,29 +427,29 @@ static inline void rcu_preempt_sleep_check(void) { }
 
 #define __rcu_access_pointer(p, local, space) \
 ({ \
-	typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
+	typeof(*(p)) *local = (typeof(*(p)) *__force)READ_ONCE(p); \
 	rcu_check_sparse(p, space); \
-	((typeof(*p) __force __kernel *)(local)); \
+	((typeof(*(p)) __force __kernel *)(local)); \
 })
 #define __rcu_dereference_check(p, local, c, space) \
 ({ \
 	/* Dependency order vs. p above. */ \
-	typeof(*p) *local = (typeof(*p) *__force)READ_ONCE(p); \
+	typeof(*(p)) *local = (typeof(*(p)) *__force)READ_ONCE(p); \
 	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
 	rcu_check_sparse(p, space); \
-	((typeof(*p) __force __kernel *)(local)); \
+	((typeof(*(p)) __force __kernel *)(local)); \
 })
 #define __rcu_dereference_protected(p, local, c, space) \
 ({ \
 	RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
 	rcu_check_sparse(p, space); \
-	((typeof(*p) __force __kernel *)(p)); \
+	((typeof(*(p)) __force __kernel *)(p)); \
 })
 #define __rcu_dereference_raw(p, local) \
 ({ \
 	/* Dependency order vs. p above. */ \
 	typeof(p) local = READ_ONCE(p); \
-	((typeof(*p) __force __kernel *)(local)); \
+	((typeof(*(p)) __force __kernel *)(local)); \
 })
 #define rcu_dereference_raw(p) __rcu_dereference_raw(p, __UNIQUE_ID(rcu))