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Date: Fri, 5 May 2023 11:39:24 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
Joel Fernandes <joel@...lfernandes.org>,
Josh Triplett <josh@...htriplett.org>,
Boqun Feng <boqun.feng@...il.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Zqiang <qiang1.zhang@...el.com>
Subject: Re: [RFC PATCH] rcu: rcupdate.h: Add missing parentheses around
macro pointer dereference
On Fri, May 05, 2023 at 09:15:12AM -0400, Mathieu Desnoyers wrote:
> On 2023-05-04 20:28, Paul E. McKenney wrote:
> > 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@...icios.com> 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@...icios.com>
> > > > Cc: "Paul E. McKenney" <paulmck@...nel.org>
> > > > Cc: Joel Fernandes <joel@...lfernandes.org>
> > > > Cc: Josh Triplett <josh@...htriplett.org>
> > > > Cc: Boqun Feng <boqun.feng@...il.com>
> > > > Cc: Steven Rostedt <rostedt@...dmis.org>
> > > > Cc: Lai Jiangshan <jiangshanlai@...il.com>
> > > > Cc: Zqiang <qiang1.zhang@...el.com>
> > > > ---
> > > > 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@...dmis.org>
> >
> > Thank you all! I applied Steve's suggested change with attribution
> > as shown below. Please let me know if I messed anything up.
>
> Hi Paul,
>
> I've done a new version of that patch which fixes other issues in rcupdate.h
> in the next round. Can you hold merging this until I remove the "RFC PATCH"
> tag please ? My goal is to gather feedback first to make sure everyone is OK
> with the overall changes across headers, so everything can become
> consistent.
Hello, Mathieu,
Very well, I have removed it.
Thanx, Paul
> Thanks,
>
> Mathieu
>
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit d3d734216c88fb7c13205dc62178ff5011da415b
> > Author: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > 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@...icios.com>
> > Cc: "Paul E. McKenney" <paulmck@...nel.org>
> > Cc: Joel Fernandes <joel@...lfernandes.org>
> > Cc: Josh Triplett <josh@...htriplett.org>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Lai Jiangshan <jiangshanlai@...il.com>
> > Cc: Zqiang <qiang1.zhang@...el.com>
> > Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> > Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> > Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> >
> > 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))
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>
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