linux-kernel - Re: [PATCH v2 06/34] cleanup: Basic compatibility with capability analysis

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Message-ID: <20250304125516.GF11590@noisy.programming.kicks-ass.net>
Date: Tue, 4 Mar 2025 13:55:16 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Marco Elver <elver@...gle.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Alexander Potapenko <glider@...gle.com>,
	Arnd Bergmann <arnd@...db.de>, Bart Van Assche <bvanassche@....org>,
	Bill Wendling <morbo@...gle.com>, Boqun Feng <boqun.feng@...il.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Ingo Molnar <mingo@...nel.org>, Jann Horn <jannh@...gle.com>,
	Jiri Slaby <jirislaby@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Jonathan Corbet <corbet@....net>,
	Josh Triplett <josh@...htriplett.org>,
	Justin Stitt <justinstitt@...gle.com>, Kees Cook <kees@...nel.org>,
	Kentaro Takeda <takedakn@...data.co.jp>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uladzislau Rezki <urezki@...il.com>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>,
	kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, rcu@...r.kernel.org,
	linux-crypto@...r.kernel.org, linux-serial@...r.kernel.org
Subject: Re: [PATCH v2 06/34] cleanup: Basic compatibility with capability
 analysis

On Tue, Mar 04, 2025 at 10:21:05AM +0100, Marco Elver wrote:
> Due to the scoped cleanup helpers used for lock guards wrapping
> acquire/release around their own constructors/destructors that store
> pointers to the passed locks in a separate struct, we currently cannot
> accurately annotate *destructors* which lock was released. While it's
> possible to annotate the constructor to say which lock was acquired,
> that alone would result in false positives claiming the lock was not
> released on function return.
> 
> Instead, to avoid false positives, we can claim that the constructor
> "asserts" that the taken lock is held. This will ensure we can still
> benefit from the analysis where scoped guards are used to protect access
> to guarded variables, while avoiding false positives. The only downside
> are false negatives where we might accidentally lock the same lock
> again:
> 
> 	raw_spin_lock(&my_lock);
> 	...
> 	guard(raw_spinlock)(&my_lock);  // no warning
> 
> Arguably, lockdep will immediately catch issues like this.
> 
> While Clang's analysis supports scoped guards in C++ [1], there's no way
> to apply this to C right now. Better support for Linux's scoped guard
> design could be added in future if deemed critical.

Would definitely be nice to have.


> @@ -383,6 +387,7 @@ static inline void *class_##_name##_lock_ptr(class_##_name##_t *_T)	\
>  
>  #define __DEFINE_LOCK_GUARD_1(_name, _type, _lock)			\
>  static inline class_##_name##_t class_##_name##_constructor(_type *l)	\
> +	__no_capability_analysis __asserts_cap(l)			\
>  {									\
>  	class_##_name##_t _t = { .lock = l }, *_T = &_t;		\
>  	_lock;								\
> @@ -391,6 +396,7 @@ static inline class_##_name##_t class_##_name##_constructor(_type *l)	\
>  
>  #define __DEFINE_LOCK_GUARD_0(_name, _lock)				\
>  static inline class_##_name##_t class_##_name##_constructor(void)	\
> +	__no_capability_analysis					\

Does this not need __asserts_cal(_lock) or somesuch?

GUARD_0 is the one used for RCU and preempt, rather sad if it doesn't
have annotations at all.