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

Marco Elver
2025-03-04T13:09:59Z

On Tue, 4 Mar 2025 at 13:55, Peter Zijlstra <peterz@infradead.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).