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Date: Fri, 5 May 2023 09:15:12 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: paulmck@...nel.org, Steven Rostedt <rostedt@...dmis.org>
Cc: 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 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.
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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