[PATCH 1/3] kcsan: Introduce KCSAN_ACCESS_ASSERT access type

[PATCH 1/3] kcsan: Introduce KCSAN_ACCESS_ASSERT access type

[Marco Elver]

The KCSAN_ACCESS_ASSERT access type may be used to introduce dummy reads
and writes to assert certain properties of synchronization logic, where
bugs could not be detected as normal data races.

For example, a variable that is only meant to be written by a single
CPU, but may be read (without locking) by other CPUs must still be
marked properly to avoid data races. However, concurrent writes,
regardless if WRITE_ONCE() or not, would be a bug. Using
kcsan_check_access(&x, sizeof(x), KCSAN_ACCESS_ASSERT) would allow
catching such bugs.

Notably, the KCSAN_ACCESS_ASSERT type disables various filters, so that
all races that KCSAN observes are reported.

Signed-off-by: Marco Elver <elver@google.com>
---
 include/linux/kcsan-checks.h |  8 +++++++-
 kernel/kcsan/core.c          | 24 +++++++++++++++++++++---
 kernel/kcsan/report.c        | 11 +++++++++++
 3 files changed, 39 insertions(+), 4 deletions(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index ef3ee233a3fa9..21b1d1f214ad5 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -6,10 +6,16 @@
 #include <linux/types.h>
 
 /*
- * Access type modifiers.
+ * ACCESS TYPE MODIFIERS
+ *
+ *   <none>: normal read access;
+ *   WRITE : write access;
+ *   ATOMIC: access is atomic;
+ *   ASSERT: access is not a regular access, but an assertion;
  */
 #define KCSAN_ACCESS_WRITE  0x1
 #define KCSAN_ACCESS_ATOMIC 0x2
+#define KCSAN_ACCESS_ASSERT 0x4
 
 /*
  * __kcsan_*: Always calls into the runtime when KCSAN is enabled. This may be used
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 82c2bef827d42..190fb5c958fe3 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -178,6 +178,14 @@ is_atomic(const volatile void *ptr, size_t size, int type)
 	if ((type & KCSAN_ACCESS_ATOMIC) != 0)
 		return true;
 
+	/*
+	 * Unless explicitly declared atomic, never consider an assertion access
+	 * as atomic. This allows using them also in atomic regions, such as
+	 * seqlocks, without implicitly changing their semantics.
+	 */
+	if ((type & KCSAN_ACCESS_ASSERT) != 0)
+		return false;
+
 	if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) &&
 	    (type & KCSAN_ACCESS_WRITE) != 0 && size <= sizeof(long) &&
 	    IS_ALIGNED((unsigned long)ptr, size))
@@ -307,6 +315,7 @@ static noinline void
 kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 {
 	const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
+	const bool is_assertion = (type & KCSAN_ACCESS_ASSERT) != 0;
 	atomic_long_t *watchpoint;
 	union {
 		u8 _1;
@@ -430,12 +439,21 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
 		 * No need to increment 'data_races' counter, as the racing
 		 * thread already did.
 		 */
-		kcsan_report(ptr, size, type, size > 8 || value_change,
-			     smp_processor_id(), KCSAN_REPORT_RACE_SIGNAL);
+
+		/*
+		 * - If we were not able to observe a value change due to size
+		 *   constraints, always assume a value change.
+		 * - If the access type is an assertion, we also always assume a
+		 *   value change to always report the race.
+		 */
+		value_change = value_change || size > 8 || is_assertion;
+
+		kcsan_report(ptr, size, type, value_change, smp_processor_id(),
+			     KCSAN_REPORT_RACE_SIGNAL);
 	} else if (value_change) {
 		/* Inferring a race, since the value should not have changed. */
 		kcsan_counter_inc(KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN);
-		if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN))
+		if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assertion)
 			kcsan_report(ptr, size, type, true,
 				     smp_processor_id(),
 				     KCSAN_REPORT_RACE_UNKNOWN_ORIGIN);
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index 7cd34285df740..938146104e046 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -178,6 +178,17 @@ static const char *get_access_type(int type)
 		return "write";
 	case KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC:
 		return "write (marked)";
+
+	/*
+	 * ASSERT variants:
+	 */
+	case KCSAN_ACCESS_ASSERT:
+	case KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_ATOMIC:
+		return "assert no writes";
+	case KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_WRITE:
+	case KCSAN_ACCESS_ASSERT | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ATOMIC:
+		return "assert no accesses";
+
 	default:
 		BUG();
 	}
-- 
2.25.0.341.g760bfbb309-goog

[PATCH 2/3] kcsan: Introduce ASSERT_EXCLUSIVE_* macros

[Marco Elver]

Introduces ASSERT_EXCLUSIVE_WRITER and ASSERT_EXCLUSIVE_ACCESS, which
may be used to assert properties of synchronization logic, where
violation cannot be detected as a normal data race.

Examples of the reports that may be generated:

    ==================================================================
    BUG: KCSAN: data-race in test_thread / test_thread

    write to 0xffffffffab3d1540 of 8 bytes by task 466 on cpu 2:
     test_thread+0x8d/0x111
     debugfs_write.cold+0x32/0x44
     ...

    assert no writes to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
     test_thread+0xa3/0x111
     debugfs_write.cold+0x32/0x44
     ...
    ==================================================================

    ==================================================================
    BUG: KCSAN: data-race in test_thread / test_thread

    assert no accesses to 0xffffffffab3d1540 of 8 bytes by task 465 on cpu 1:
     test_thread+0xb9/0x111
     debugfs_write.cold+0x32/0x44
     ...

    read to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
     test_thread+0x77/0x111
     debugfs_write.cold+0x32/0x44
     ...
    ==================================================================

Signed-off-by: Marco Elver <elver@google.com>
Suggested-by: Paul E. McKenney <paulmck@kernel.org>
---

Please let me know if the names make sense, given they do not include a
KCSAN_ prefix.

The names are unique across the kernel. I wouldn't expect another macro
with the same name but different semantics to pop up any time soon. If
there is a dual use to these macros (e.g. another tool that could hook
into it), we could also move it elsewhere (include/linux/compiler.h?).

We can also revisit the original suggestion of WRITE_ONCE_EXCLUSIVE(),
if it is something that'd be used very widely. It'd be straightforward
to add with the help of these macros, but would need to be added to
include/linux/compiler.h.
---
 include/linux/kcsan-checks.h | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 21b1d1f214ad5..1a7b51e516335 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -96,4 +96,38 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
 	kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
 #endif
 
+/**
+ * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
+ *
+ * Assert that there are no other threads writing @var; other readers are
+ * allowed. This assertion can be used to specify properties of synchronization
+ * logic, where violation cannot be detected as a normal data race.
+ *
+ * For example, if a per-CPU variable is only meant to be written by a single
+ * CPU, but may be read from other CPUs; in this case, reads and writes must be
+ * marked properly, however, if an off-CPU WRITE_ONCE() races with the owning
+ * CPU's WRITE_ONCE(), would not constitute a data race but could be a harmful
+ * race condition. Using this macro allows specifying this property in the code
+ * and catch such bugs.
+ *
+ * @var variable to assert on
+ */
+#define ASSERT_EXCLUSIVE_WRITER(var)                                           \
+	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
+
+/**
+ * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
+ *
+ * Assert that no other thread is accessing @var (no readers nor writers). This
+ * assertion can be used to specify properties of synchronization logic, where
+ * violation cannot be detected as a normal data race.
+ *
+ * For example, if a variable is not read nor written by the current thread, nor
+ * should it be touched by any other threads during the current execution phase.
+ *
+ * @var variable to assert on
+ */
+#define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
+	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
+
 #endif /* _LINUX_KCSAN_CHECKS_H */
-- 
2.25.0.341.g760bfbb309-goog

Re: [PATCH 2/3] kcsan: Introduce ASSERT_EXCLUSIVE_* macros

[Paul E. McKenney]

On Wed, Feb 05, 2020 at 09:43:32PM +0100, Marco Elver wrote:
> Introduces ASSERT_EXCLUSIVE_WRITER and ASSERT_EXCLUSIVE_ACCESS, which
> may be used to assert properties of synchronization logic, where
> violation cannot be detected as a normal data race.
> 
> Examples of the reports that may be generated:
> 
>     ==================================================================
>     BUG: KCSAN: data-race in test_thread / test_thread
> 
>     write to 0xffffffffab3d1540 of 8 bytes by task 466 on cpu 2:
>      test_thread+0x8d/0x111
>      debugfs_write.cold+0x32/0x44
>      ...
> 
>     assert no writes to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
>      test_thread+0xa3/0x111
>      debugfs_write.cold+0x32/0x44
>      ...
>     ==================================================================
> 
>     ==================================================================
>     BUG: KCSAN: data-race in test_thread / test_thread
> 
>     assert no accesses to 0xffffffffab3d1540 of 8 bytes by task 465 on cpu 1:
>      test_thread+0xb9/0x111
>      debugfs_write.cold+0x32/0x44
>      ...
> 
>     read to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
>      test_thread+0x77/0x111
>      debugfs_write.cold+0x32/0x44
>      ...
>     ==================================================================
> 
> Signed-off-by: Marco Elver <elver@google.com>
> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> 
> Please let me know if the names make sense, given they do not include a
> KCSAN_ prefix.

I am OK with this, but there might well be some bikeshedding later on.
Which should not be a real problem, irritating though it might be.

> The names are unique across the kernel. I wouldn't expect another macro
> with the same name but different semantics to pop up any time soon. If
> there is a dual use to these macros (e.g. another tool that could hook
> into it), we could also move it elsewhere (include/linux/compiler.h?).
> 
> We can also revisit the original suggestion of WRITE_ONCE_EXCLUSIVE(),
> if it is something that'd be used very widely. It'd be straightforward
> to add with the help of these macros, but would need to be added to
> include/linux/compiler.h.

A more definite use case for ASSERT_EXCLUSIVE_ACCESS() is a
reference-counting algorithm where exclusive access is expected after
a successful atomic_dec_and_test().  Any objection to making the
docbook header use that example?  I believe that a more familiar
example would help people see the point of all this.  ;-)

I am queueing these as-is for review and testing, but please feel free
to send updated versions.  Easy to do the replacement!

And you knew that this was coming...  It looks to me that I can
do something like this:

	struct foo {
		int a;
		char b;
		long c;
		atomic_t refctr;
	};

	void do_a_foo(struct foo *fp)
	{
		if (atomic_dec_and_test(&fp->refctr)) {
			ASSERT_EXCLUSIVE_ACCESS(*fp);
			safely_dispose_of(fp);
		}
	}

Does that work, or is it necessary to assert for each field separately?

							Thanx, Paul

> ---
>  include/linux/kcsan-checks.h | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> index 21b1d1f214ad5..1a7b51e516335 100644
> --- a/include/linux/kcsan-checks.h
> +++ b/include/linux/kcsan-checks.h
> @@ -96,4 +96,38 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
>  	kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
>  #endif
>  
> +/**
> + * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
> + *
> + * Assert that there are no other threads writing @var; other readers are
> + * allowed. This assertion can be used to specify properties of synchronization
> + * logic, where violation cannot be detected as a normal data race.
> + *
> + * For example, if a per-CPU variable is only meant to be written by a single
> + * CPU, but may be read from other CPUs; in this case, reads and writes must be
> + * marked properly, however, if an off-CPU WRITE_ONCE() races with the owning
> + * CPU's WRITE_ONCE(), would not constitute a data race but could be a harmful
> + * race condition. Using this macro allows specifying this property in the code
> + * and catch such bugs.
> + *
> + * @var variable to assert on
> + */
> +#define ASSERT_EXCLUSIVE_WRITER(var)                                           \
> +	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
> +
> +/**
> + * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
> + *
> + * Assert that no other thread is accessing @var (no readers nor writers). This
> + * assertion can be used to specify properties of synchronization logic, where
> + * violation cannot be detected as a normal data race.
> + *
> + * For example, if a variable is not read nor written by the current thread, nor
> + * should it be touched by any other threads during the current execution phase.
> + *
> + * @var variable to assert on
> + */
> +#define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
> +	__kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
> +
>  #endif /* _LINUX_KCSAN_CHECKS_H */
> -- 
> 2.25.0.341.g760bfbb309-goog
> 

Re: [PATCH 2/3] kcsan: Introduce ASSERT_EXCLUSIVE_* macros

[Marco Elver]

On Wed, 5 Feb 2020 at 22:33, Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Wed, Feb 05, 2020 at 09:43:32PM +0100, Marco Elver wrote:
> > Introduces ASSERT_EXCLUSIVE_WRITER and ASSERT_EXCLUSIVE_ACCESS, which
> > may be used to assert properties of synchronization logic, where
> > violation cannot be detected as a normal data race.
> >
> > Examples of the reports that may be generated:
> >
> >     ==================================================================
> >     BUG: KCSAN: data-race in test_thread / test_thread
> >
> >     write to 0xffffffffab3d1540 of 8 bytes by task 466 on cpu 2:
> >      test_thread+0x8d/0x111
> >      debugfs_write.cold+0x32/0x44
> >      ...
> >
> >     assert no writes to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
> >      test_thread+0xa3/0x111
> >      debugfs_write.cold+0x32/0x44
> >      ...
> >     ==================================================================
> >
> >     ==================================================================
> >     BUG: KCSAN: data-race in test_thread / test_thread
> >
> >     assert no accesses to 0xffffffffab3d1540 of 8 bytes by task 465 on cpu 1:
> >      test_thread+0xb9/0x111
> >      debugfs_write.cold+0x32/0x44
> >      ...
> >
> >     read to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
> >      test_thread+0x77/0x111
> >      debugfs_write.cold+0x32/0x44
> >      ...
> >     ==================================================================
> >
> > Signed-off-by: Marco Elver <elver@google.com>
> > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >
> > Please let me know if the names make sense, given they do not include a
> > KCSAN_ prefix.
>
> I am OK with this, but there might well be some bikeshedding later on.
> Which should not be a real problem, irritating though it might be.
>
> > The names are unique across the kernel. I wouldn't expect another macro
> > with the same name but different semantics to pop up any time soon. If
> > there is a dual use to these macros (e.g. another tool that could hook
> > into it), we could also move it elsewhere (include/linux/compiler.h?).
> >
> > We can also revisit the original suggestion of WRITE_ONCE_EXCLUSIVE(),
> > if it is something that'd be used very widely. It'd be straightforward
> > to add with the help of these macros, but would need to be added to
> > include/linux/compiler.h.
>
> A more definite use case for ASSERT_EXCLUSIVE_ACCESS() is a
> reference-counting algorithm where exclusive access is expected after
> a successful atomic_dec_and_test().  Any objection to making the
> docbook header use that example?  I believe that a more familiar
> example would help people see the point of all this.  ;-)

Happy to update the example -- I'll send it tomorrow.

> I am queueing these as-is for review and testing, but please feel free
> to send updated versions.  Easy to do the replacement!

Thank you!

> And you knew that this was coming...  It looks to me that I can
> do something like this:
>
>         struct foo {
>                 int a;
>                 char b;
>                 long c;
>                 atomic_t refctr;
>         };
>
>         void do_a_foo(struct foo *fp)
>         {
>                 if (atomic_dec_and_test(&fp->refctr)) {
>                         ASSERT_EXCLUSIVE_ACCESS(*fp);
>                         safely_dispose_of(fp);
>                 }
>         }
>
> Does that work, or is it necessary to assert for each field separately?

That works just fine, and will check for races on the whole struct.

Thanks,
-- Marco

>                                                         Thanx, Paul
>
> > ---
> >  include/linux/kcsan-checks.h | 34 ++++++++++++++++++++++++++++++++++
> >  1 file changed, 34 insertions(+)
> >
> > diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> > index 21b1d1f214ad5..1a7b51e516335 100644
> > --- a/include/linux/kcsan-checks.h
> > +++ b/include/linux/kcsan-checks.h
> > @@ -96,4 +96,38 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> >       kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
> >  #endif
> >
> > +/**
> > + * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
> > + *
> > + * Assert that there are no other threads writing @var; other readers are
> > + * allowed. This assertion can be used to specify properties of synchronization
> > + * logic, where violation cannot be detected as a normal data race.
> > + *
> > + * For example, if a per-CPU variable is only meant to be written by a single
> > + * CPU, but may be read from other CPUs; in this case, reads and writes must be
> > + * marked properly, however, if an off-CPU WRITE_ONCE() races with the owning
> > + * CPU's WRITE_ONCE(), would not constitute a data race but could be a harmful
> > + * race condition. Using this macro allows specifying this property in the code
> > + * and catch such bugs.
> > + *
> > + * @var variable to assert on
> > + */
> > +#define ASSERT_EXCLUSIVE_WRITER(var)                                           \
> > +     __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
> > +
> > +/**
> > + * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
> > + *
> > + * Assert that no other thread is accessing @var (no readers nor writers). This
> > + * assertion can be used to specify properties of synchronization logic, where
> > + * violation cannot be detected as a normal data race.
> > + *
> > + * For example, if a variable is not read nor written by the current thread, nor
> > + * should it be touched by any other threads during the current execution phase.
> > + *
> > + * @var variable to assert on
> > + */
> > +#define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
> > +     __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
> > +
> >  #endif /* _LINUX_KCSAN_CHECKS_H */
> > --
> > 2.25.0.341.g760bfbb309-goog
> >
>
> --
> You received this message because you are subscribed to the Google Groups "kasan-dev" group.
> To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+unsubscribe@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/kasan-dev/20200205213302.GA2935%40paulmck-ThinkPad-P72.

Re: [PATCH 2/3] kcsan: Introduce ASSERT_EXCLUSIVE_* macros

[Paul E. McKenney]

On Wed, Feb 05, 2020 at 10:48:14PM +0100, Marco Elver wrote:
> On Wed, 5 Feb 2020 at 22:33, Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > On Wed, Feb 05, 2020 at 09:43:32PM +0100, Marco Elver wrote:
> > > Introduces ASSERT_EXCLUSIVE_WRITER and ASSERT_EXCLUSIVE_ACCESS, which
> > > may be used to assert properties of synchronization logic, where
> > > violation cannot be detected as a normal data race.
> > >
> > > Examples of the reports that may be generated:
> > >
> > >     ==================================================================
> > >     BUG: KCSAN: data-race in test_thread / test_thread
> > >
> > >     write to 0xffffffffab3d1540 of 8 bytes by task 466 on cpu 2:
> > >      test_thread+0x8d/0x111
> > >      debugfs_write.cold+0x32/0x44
> > >      ...
> > >
> > >     assert no writes to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
> > >      test_thread+0xa3/0x111
> > >      debugfs_write.cold+0x32/0x44
> > >      ...
> > >     ==================================================================
> > >
> > >     ==================================================================
> > >     BUG: KCSAN: data-race in test_thread / test_thread
> > >
> > >     assert no accesses to 0xffffffffab3d1540 of 8 bytes by task 465 on cpu 1:
> > >      test_thread+0xb9/0x111
> > >      debugfs_write.cold+0x32/0x44
> > >      ...
> > >
> > >     read to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
> > >      test_thread+0x77/0x111
> > >      debugfs_write.cold+0x32/0x44
> > >      ...
> > >     ==================================================================
> > >
> > > Signed-off-by: Marco Elver <elver@google.com>
> > > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > > ---
> > >
> > > Please let me know if the names make sense, given they do not include a
> > > KCSAN_ prefix.
> >
> > I am OK with this, but there might well be some bikeshedding later on.
> > Which should not be a real problem, irritating though it might be.
> >
> > > The names are unique across the kernel. I wouldn't expect another macro
> > > with the same name but different semantics to pop up any time soon. If
> > > there is a dual use to these macros (e.g. another tool that could hook
> > > into it), we could also move it elsewhere (include/linux/compiler.h?).
> > >
> > > We can also revisit the original suggestion of WRITE_ONCE_EXCLUSIVE(),
> > > if it is something that'd be used very widely. It'd be straightforward
> > > to add with the help of these macros, but would need to be added to
> > > include/linux/compiler.h.
> >
> > A more definite use case for ASSERT_EXCLUSIVE_ACCESS() is a
> > reference-counting algorithm where exclusive access is expected after
> > a successful atomic_dec_and_test().  Any objection to making the
> > docbook header use that example?  I believe that a more familiar
> > example would help people see the point of all this.  ;-)
> 
> Happy to update the example -- I'll send it tomorrow.

Sounds great!

> > I am queueing these as-is for review and testing, but please feel free
> > to send updated versions.  Easy to do the replacement!
> 
> Thank you!
> 
> > And you knew that this was coming...  It looks to me that I can
> > do something like this:
> >
> >         struct foo {
> >                 int a;
> >                 char b;
> >                 long c;
> >                 atomic_t refctr;
> >         };
> >
> >         void do_a_foo(struct foo *fp)
> >         {
> >                 if (atomic_dec_and_test(&fp->refctr)) {
> >                         ASSERT_EXCLUSIVE_ACCESS(*fp);
> >                         safely_dispose_of(fp);
> >                 }
> >         }
> >
> > Does that work, or is it necessary to assert for each field separately?
> 
> That works just fine, and will check for races on the whole struct.

Nice!!!

							Thanx, Paul

> Thanks,
> -- Marco
> 
> >                                                         Thanx, Paul
> >
> > > ---
> > >  include/linux/kcsan-checks.h | 34 ++++++++++++++++++++++++++++++++++
> > >  1 file changed, 34 insertions(+)
> > >
> > > diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> > > index 21b1d1f214ad5..1a7b51e516335 100644
> > > --- a/include/linux/kcsan-checks.h
> > > +++ b/include/linux/kcsan-checks.h
> > > @@ -96,4 +96,38 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> > >       kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
> > >  #endif
> > >
> > > +/**
> > > + * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
> > > + *
> > > + * Assert that there are no other threads writing @var; other readers are
> > > + * allowed. This assertion can be used to specify properties of synchronization
> > > + * logic, where violation cannot be detected as a normal data race.
> > > + *
> > > + * For example, if a per-CPU variable is only meant to be written by a single
> > > + * CPU, but may be read from other CPUs; in this case, reads and writes must be
> > > + * marked properly, however, if an off-CPU WRITE_ONCE() races with the owning
> > > + * CPU's WRITE_ONCE(), would not constitute a data race but could be a harmful
> > > + * race condition. Using this macro allows specifying this property in the code
> > > + * and catch such bugs.
> > > + *
> > > + * @var variable to assert on
> > > + */
> > > +#define ASSERT_EXCLUSIVE_WRITER(var)                                           \
> > > +     __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
> > > +
> > > +/**
> > > + * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
> > > + *
> > > + * Assert that no other thread is accessing @var (no readers nor writers). This
> > > + * assertion can be used to specify properties of synchronization logic, where
> > > + * violation cannot be detected as a normal data race.
> > > + *
> > > + * For example, if a variable is not read nor written by the current thread, nor
> > > + * should it be touched by any other threads during the current execution phase.
> > > + *
> > > + * @var variable to assert on
> > > + */
> > > +#define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
> > > +     __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
> > > +
> > >  #endif /* _LINUX_KCSAN_CHECKS_H */
> > > --
> > > 2.25.0.341.g760bfbb309-goog
> > >
> >
> > --
> > You received this message because you are subscribed to the Google Groups "kasan-dev" group.
> > To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+unsubscribe@googlegroups.com.
> > To view this discussion on the web visit https://groups.google.com/d/msgid/kasan-dev/20200205213302.GA2935%40paulmck-ThinkPad-P72.

Re: [PATCH 2/3] kcsan: Introduce ASSERT_EXCLUSIVE_* macros

[Marco Elver]

On Wed, 5 Feb 2020 at 23:04, Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Wed, Feb 05, 2020 at 10:48:14PM +0100, Marco Elver wrote:
> > On Wed, 5 Feb 2020 at 22:33, Paul E. McKenney <paulmck@kernel.org> wrote:
> > >
> > > On Wed, Feb 05, 2020 at 09:43:32PM +0100, Marco Elver wrote:
> > > > Introduces ASSERT_EXCLUSIVE_WRITER and ASSERT_EXCLUSIVE_ACCESS, which
> > > > may be used to assert properties of synchronization logic, where
> > > > violation cannot be detected as a normal data race.
> > > >
> > > > Examples of the reports that may be generated:
> > > >
> > > >     ==================================================================
> > > >     BUG: KCSAN: data-race in test_thread / test_thread
> > > >
> > > >     write to 0xffffffffab3d1540 of 8 bytes by task 466 on cpu 2:
> > > >      test_thread+0x8d/0x111
> > > >      debugfs_write.cold+0x32/0x44
> > > >      ...
> > > >
> > > >     assert no writes to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
> > > >      test_thread+0xa3/0x111
> > > >      debugfs_write.cold+0x32/0x44
> > > >      ...
> > > >     ==================================================================
> > > >
> > > >     ==================================================================
> > > >     BUG: KCSAN: data-race in test_thread / test_thread
> > > >
> > > >     assert no accesses to 0xffffffffab3d1540 of 8 bytes by task 465 on cpu 1:
> > > >      test_thread+0xb9/0x111
> > > >      debugfs_write.cold+0x32/0x44
> > > >      ...
> > > >
> > > >     read to 0xffffffffab3d1540 of 8 bytes by task 464 on cpu 0:
> > > >      test_thread+0x77/0x111
> > > >      debugfs_write.cold+0x32/0x44
> > > >      ...
> > > >     ==================================================================
> > > >
> > > > Signed-off-by: Marco Elver <elver@google.com>
> > > > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > > > ---
> > > >
> > > > Please let me know if the names make sense, given they do not include a
> > > > KCSAN_ prefix.
> > >
> > > I am OK with this, but there might well be some bikeshedding later on.
> > > Which should not be a real problem, irritating though it might be.
> > >
> > > > The names are unique across the kernel. I wouldn't expect another macro
> > > > with the same name but different semantics to pop up any time soon. If
> > > > there is a dual use to these macros (e.g. another tool that could hook
> > > > into it), we could also move it elsewhere (include/linux/compiler.h?).
> > > >
> > > > We can also revisit the original suggestion of WRITE_ONCE_EXCLUSIVE(),
> > > > if it is something that'd be used very widely. It'd be straightforward
> > > > to add with the help of these macros, but would need to be added to
> > > > include/linux/compiler.h.
> > >
> > > A more definite use case for ASSERT_EXCLUSIVE_ACCESS() is a
> > > reference-counting algorithm where exclusive access is expected after
> > > a successful atomic_dec_and_test().  Any objection to making the
> > > docbook header use that example?  I believe that a more familiar
> > > example would help people see the point of all this.  ;-)
> >
> > Happy to update the example -- I'll send it tomorrow.
>
> Sounds great!

v2 sent: http://lkml.kernel.org/r/20200206154626.243230-1-elver@google.com

Thanks,
-- Marco

> > > I am queueing these as-is for review and testing, but please feel free
> > > to send updated versions.  Easy to do the replacement!
> >
> > Thank you!
> >
> > > And you knew that this was coming...  It looks to me that I can
> > > do something like this:
> > >
> > >         struct foo {
> > >                 int a;
> > >                 char b;
> > >                 long c;
> > >                 atomic_t refctr;
> > >         };
> > >
> > >         void do_a_foo(struct foo *fp)
> > >         {
> > >                 if (atomic_dec_and_test(&fp->refctr)) {
> > >                         ASSERT_EXCLUSIVE_ACCESS(*fp);
> > >                         safely_dispose_of(fp);
> > >                 }
> > >         }
> > >
> > > Does that work, or is it necessary to assert for each field separately?
> >
> > That works just fine, and will check for races on the whole struct.
>
> Nice!!!
>
>                                                         Thanx, Paul
>
> > Thanks,
> > -- Marco
> >
> > >                                                         Thanx, Paul
> > >
> > > > ---
> > > >  include/linux/kcsan-checks.h | 34 ++++++++++++++++++++++++++++++++++
> > > >  1 file changed, 34 insertions(+)
> > > >
> > > > diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
> > > > index 21b1d1f214ad5..1a7b51e516335 100644
> > > > --- a/include/linux/kcsan-checks.h
> > > > +++ b/include/linux/kcsan-checks.h
> > > > @@ -96,4 +96,38 @@ static inline void kcsan_check_access(const volatile void *ptr, size_t size,
> > > >       kcsan_check_access(ptr, size, KCSAN_ACCESS_ATOMIC | KCSAN_ACCESS_WRITE)
> > > >  #endif
> > > >
> > > > +/**
> > > > + * ASSERT_EXCLUSIVE_WRITER - assert no other threads are writing @var
> > > > + *
> > > > + * Assert that there are no other threads writing @var; other readers are
> > > > + * allowed. This assertion can be used to specify properties of synchronization
> > > > + * logic, where violation cannot be detected as a normal data race.
> > > > + *
> > > > + * For example, if a per-CPU variable is only meant to be written by a single
> > > > + * CPU, but may be read from other CPUs; in this case, reads and writes must be
> > > > + * marked properly, however, if an off-CPU WRITE_ONCE() races with the owning
> > > > + * CPU's WRITE_ONCE(), would not constitute a data race but could be a harmful
> > > > + * race condition. Using this macro allows specifying this property in the code
> > > > + * and catch such bugs.
> > > > + *
> > > > + * @var variable to assert on
> > > > + */
> > > > +#define ASSERT_EXCLUSIVE_WRITER(var)                                           \
> > > > +     __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_ASSERT)
> > > > +
> > > > +/**
> > > > + * ASSERT_EXCLUSIVE_ACCESS - assert no other threads are accessing @var
> > > > + *
> > > > + * Assert that no other thread is accessing @var (no readers nor writers). This
> > > > + * assertion can be used to specify properties of synchronization logic, where
> > > > + * violation cannot be detected as a normal data race.
> > > > + *
> > > > + * For example, if a variable is not read nor written by the current thread, nor
> > > > + * should it be touched by any other threads during the current execution phase.
> > > > + *
> > > > + * @var variable to assert on
> > > > + */
> > > > +#define ASSERT_EXCLUSIVE_ACCESS(var)                                           \
> > > > +     __kcsan_check_access(&(var), sizeof(var), KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT)
> > > > +
> > > >  #endif /* _LINUX_KCSAN_CHECKS_H */
> > > > --
> > > > 2.25.0.341.g760bfbb309-goog
> > > >
> > >
> > > --
> > > You received this message because you are subscribed to the Google Groups "kasan-dev" group.
> > > To unsubscribe from this group and stop receiving emails from it, send an email to kasan-dev+unsubscribe@googlegroups.com.
> > > To view this discussion on the web visit https://groups.google.com/d/msgid/kasan-dev/20200205213302.GA2935%40paulmck-ThinkPad-P72.

[PATCH 3/3] kcsan: Add test to generate conflicts via debugfs

[Marco Elver]

Add 'test=<iters>' option to KCSAN's debugfs interface to invoke KCSAN
checks on a dummy variable. By writing 'test=<iters>' to the debugfs
file from multiple tasks, we can generate real conflicts, and trigger
data race reports.

Signed-off-by: Marco Elver <elver@google.com>
---
 kernel/kcsan/debugfs.c | 51 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 46 insertions(+), 5 deletions(-)

diff --git a/kernel/kcsan/debugfs.c b/kernel/kcsan/debugfs.c
index bec42dab32ee8..5733f51a6e2c7 100644
--- a/kernel/kcsan/debugfs.c
+++ b/kernel/kcsan/debugfs.c
@@ -6,6 +6,7 @@
 #include <linux/debugfs.h>
 #include <linux/init.h>
 #include <linux/kallsyms.h>
+#include <linux/sched.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
 #include <linux/sort.h>
@@ -68,9 +69,9 @@ void kcsan_counter_dec(enum kcsan_counter_id id)
 /*
  * The microbenchmark allows benchmarking KCSAN core runtime only. To run
  * multiple threads, pipe 'microbench=<iters>' from multiple tasks into the
- * debugfs file.
+ * debugfs file. This will not generate any conflicts, and tests fast-path only.
  */
-static void microbenchmark(unsigned long iters)
+static noinline void microbenchmark(unsigned long iters)
 {
 	cycles_t cycles;
 
@@ -80,18 +81,52 @@ static void microbenchmark(unsigned long iters)
 	while (iters--) {
 		/*
 		 * We can run this benchmark from multiple tasks; this address
-		 * calculation increases likelyhood of some accesses overlapping
-		 * (they still won't conflict because all are reads).
+		 * calculation increases likelyhood of some accesses
+		 * overlapping. Make the access type an atomic read, to never
+		 * set up watchpoints and test the fast-path only.
 		 */
 		unsigned long addr =
 			iters % (CONFIG_KCSAN_NUM_WATCHPOINTS * PAGE_SIZE);
-		__kcsan_check_read((void *)addr, sizeof(long));
+		__kcsan_check_access((void *)addr, sizeof(long), KCSAN_ACCESS_ATOMIC);
 	}
 	cycles = get_cycles() - cycles;
 
 	pr_info("KCSAN: %s end   | cycles: %llu\n", __func__, cycles);
 }
 
+/*
+ * Simple test to create conflicting accesses. Write 'test=<iters>' to KCSAN's
+ * debugfs file from multiple tasks to generate real conflicts and show reports.
+ */
+static long test_dummy;
+static noinline void test_thread(unsigned long iters)
+{
+	const struct kcsan_ctx ctx_save = current->kcsan_ctx;
+	cycles_t cycles;
+
+	/* We may have been called from an atomic region; reset context. */
+	memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
+
+	pr_info("KCSAN: %s begin | iters: %lu\n", __func__, iters);
+
+	cycles = get_cycles();
+	while (iters--) {
+		__kcsan_check_read(&test_dummy, sizeof(test_dummy));
+		__kcsan_check_write(&test_dummy, sizeof(test_dummy));
+		ASSERT_EXCLUSIVE_WRITER(test_dummy);
+		ASSERT_EXCLUSIVE_ACCESS(test_dummy);
+
+		/* not actually instrumented */
+		WRITE_ONCE(test_dummy, iters);  /* to observe value-change */
+	}
+	cycles = get_cycles() - cycles;
+
+	pr_info("KCSAN: %s end   | cycles: %llu\n", __func__, cycles);
+
+	/* restore context */
+	current->kcsan_ctx = ctx_save;
+}
+
 static int cmp_filterlist_addrs(const void *rhs, const void *lhs)
 {
 	const unsigned long a = *(const unsigned long *)rhs;
@@ -241,6 +276,12 @@ debugfs_write(struct file *file, const char __user *buf, size_t count, loff_t *o
 		if (kstrtoul(&arg[sizeof("microbench=") - 1], 0, &iters))
 			return -EINVAL;
 		microbenchmark(iters);
+	} else if (!strncmp(arg, "test=", sizeof("test=") - 1)) {
+		unsigned long iters;
+
+		if (kstrtoul(&arg[sizeof("test=") - 1], 0, &iters))
+			return -EINVAL;
+		test_thread(iters);
 	} else if (!strcmp(arg, "whitelist")) {
 		set_report_filterlist_whitelist(true);
 	} else if (!strcmp(arg, "blacklist")) {
-- 
2.25.0.341.g760bfbb309-goog