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

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Message-ID: <CANpmjNNNB8zQJKZaby8KNu8PdAJDufcia+sa2RajWm6Bd2TC4A@mail.gmail.com>
Date: Tue, 4 Mar 2025 14:09:21 +0100
From: Marco Elver <elver@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
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, 4 Mar 2025 at 13:55, Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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.

Once we have the basic infra here, I think it'll be easier to push for
these improvements. It's not entirely up to me, and we have to
coordinate with the Clang maintainers. Definitely is on the list.

> > @@ -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.

This is solved later in the series where we need it for RCU:
https://lore.kernel.org/all/20250304092417.2873893-15-elver@google.com/

We can't add this to all GUARD_0, because not all will be for
capability-enabled structs. Instead I added a helper to add the
necessary annotations where needed (see DECLARE_LOCK_GUARD_0_ATTRS).