linux-toradex.git/include/linux/kcsan-checks.h, branch v5.10-rc2

kcsan: Support compounded read-write instrumentation

2020-08-24T22:09:32+00:00

Add support for compounded read-write instrumentation if supported by
the compiler. Adds the necessary instrumentation functions, and a new
type which is used to generate a more descriptive report.

Furthermore, such compounded memory access instrumentation is excluded
from the "assume aligned writes up to word size are atomic" rule,
because we cannot assume that the compiler emits code that is atomic for
compound ops.

LLVM/Clang added support for the feature in:
https://github.com/llvm/llvm-project/commit/785d41a261d136b64ab6c15c5d35f2adc5ad53e3

The new instrumentation is emitted for sets of memory accesses in the
same basic block to the same address with at least one read appearing
before a write. These typically result from compound operations such as
++, --, +=, -=, |=, &=, etc. but also equivalent forms such as "var =
var + 1". Where the compiler determines that it is equivalent to emit a
call to a single __tsan_read_write instead of separate __tsan_read and
__tsan_write, we can then benefit from improved performance and better
reporting for such access patterns.

The new reports now show that the ops are both reads and writes, for
example:

	read-write to 0xffffffff90548a38 of 8 bytes by task 143 on cpu 3:
	 test_kernel_rmw_array+0x45/0xa0
	 access_thread+0x71/0xb0
	 kthread+0x21e/0x240
	 ret_from_fork+0x22/0x30

	read-write to 0xffffffff90548a38 of 8 bytes by task 144 on cpu 2:
	 test_kernel_rmw_array+0x45/0xa0
	 access_thread+0x71/0xb0
	 kthread+0x21e/0x240
	 ret_from_fork+0x22/0x30

Acked-by: Peter Zijlstra (Intel) 
Signed-off-by: Marco Elver 
Signed-off-by: Paul E. McKenney

kcsan: fix a kernel-doc warning

2020-06-26T16:01:08+00:00

One of the kernel-doc markups there have two "note" sections:

	./include/linux/kcsan-checks.h:346: warning: duplicate section name 'Note'

While this is not the case here, duplicated sections can cause
build issues on Sphinx. So, let's change the notes section
to use, instead, a list for those 2 notes at the same function.

Signed-off-by: Mauro Carvalho Chehab 
Acked-by: Marco Elver 
Link: https://lore.kernel.org/r/20f7995fab2ba85ce723203e9a7c822a55cca2af.1592895969.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet

kcsan: Add __kcsan_{enable,disable}_current() variants

2020-05-06T17:58:46+00:00

The __kcsan_{enable,disable}_current() variants only call into KCSAN if
KCSAN is enabled for the current compilation unit. Note: This is
typically not what we want, as we usually want to ensure that even calls
into other functions still have KCSAN disabled.

These variants may safely be used in header files that are shared
between regular kernel code and code that does not link the KCSAN
runtime.

Signed-off-by: Marco Elver 
Signed-off-by: Paul E. McKenney

kcsan: Move kcsan_{disable,enable}_current() to kcsan-checks.h

2020-04-14T00:18:14+00:00

Both affect access checks, and should therefore be in kcsan-checks.h.
This is in preparation to use these in compiler.h.

Acked-by: Will Deacon 
Signed-off-by: Marco Elver 
Signed-off-by: Paul E. McKenney

kcsan: Introduce scoped ASSERT_EXCLUSIVE macros

2020-04-14T00:18:13+00:00

Introduce ASSERT_EXCLUSIVE_*_SCOPED(), which provide an intuitive
interface to use the scoped-access feature, without having to explicitly
mark the start and end of the desired scope. Basing duration of the
checks on scope avoids accidental misuse and resulting false positives,
which may be hard to debug. See added comments for usage.

The macros are implemented using __attribute__((__cleanup__(func))),
which is supported by all compilers that currently support KCSAN.

Suggested-by: Boqun Feng 
Suggested-by: Paul E. McKenney 
Signed-off-by: Marco Elver 
Signed-off-by: Paul E. McKenney

kcsan: Add support for scoped accesses

2020-04-14T00:18:11+00:00

This adds support for scoped accesses, where the memory range is checked
for the duration of the scope. The feature is implemented by inserting
the relevant access information into a list of scoped accesses for
the current execution context, which are then checked (until removed)
on every call (through instrumentation) into the KCSAN runtime.

An alternative, more complex, implementation could set up a watchpoint for
the scoped access, and keep the watchpoint set up. This, however, would
require first exposing a handle to the watchpoint, as well as dealing
with cases such as accesses by the same thread while the watchpoint is
still set up (and several more cases). It is also doubtful if this would
provide any benefit, since the majority of delay where the watchpoint
is set up is likely due to the injected delays by KCSAN.  Therefore,
the implementation in this patch is simpler and avoids hurting KCSAN's
main use-case (normal data race detection); it also implicitly increases
scoped-access race-detection-ability due to increased probability of
setting up watchpoints by repeatedly calling __kcsan_check_access()
throughout the scope of the access.

The implementation required adding an additional conditional branch to
the fast-path. However, the microbenchmark showed a *speedup* of ~5%
on the fast-path. This appears to be due to subtly improved codegen by
GCC from moving get_ctx() and associated load of preempt_count earlier.

Suggested-by: Boqun Feng 
Suggested-by: Paul E. McKenney 
Signed-off-by: Marco Elver 
Signed-off-by: Paul E. McKenney

kcsan: Update API documentation in kcsan-checks.h

2020-03-25T16:56:00+00:00

Update the API documentation for ASSERT_EXCLUSIVE_* macros and make them
generate readable documentation for the code examples.

All @variable short summaries were missing ':', which was updated for
the whole file.

Tested with "make htmldocs".

Signed-off-by: Marco Elver 
Signed-off-by: Paul E. McKenney

kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)

2020-03-21T08:44:14+00:00

This introduces ASSERT_EXCLUSIVE_BITS(var, mask).
ASSERT_EXCLUSIVE_BITS(var, mask) will cause KCSAN to assume that the
following access is safe w.r.t. data races (however, please see the
docbook comment for disclaimer here).

For more context on why this was considered necessary, please see:

  http://lkml.kernel.org/r/1580995070-25139-1-git-send-email-cai@lca.pw

In particular, before this patch, data races between reads (that use
@mask bits of an access that should not be modified concurrently) and
writes (that change ~@mask bits not used by the readers) would have been
annotated with "data_race()" (or "READ_ONCE()"). However, doing so would
then hide real problems: we would no longer be able to detect harmful
races between reads to @mask bits and writes to @mask bits.

Therefore, by using ASSERT_EXCLUSIVE_BITS(var, mask), we accomplish:

  1. Avoid proliferation of specific macros at the call sites: by
     including a single mask in the argument list, we can use the same
     macro in a wide variety of call sites, regardless of how and which
     bits in a field each call site actually accesses.

  2. The existing code does not need to be modified (although READ_ONCE()
     may still be advisable if we cannot prove that the data race is
     always safe).

  3. We catch bugs where the exclusive bits are modified concurrently.

  4. We document properties of the current code.

Acked-by: John Hubbard 
Signed-off-by: Marco Elver 
Signed-off-by: Paul E. McKenney 
Signed-off-by: Ingo Molnar 
Cc: David Hildenbrand 
Cc: Jan Kara 
Cc: Qian Cai

kcsan: Add kcsan_set_access_mask() support

2020-03-21T08:44:08+00:00

When setting up an access mask with kcsan_set_access_mask(), KCSAN will
only report races if concurrent changes to bits set in access_mask are
observed. Conveying access_mask via a separate call avoids introducing
overhead in the common-case fast-path.

Acked-by: John Hubbard 
Signed-off-by: Marco Elver 
Signed-off-by: Paul E. McKenney 
Signed-off-by: Ingo Molnar

kcsan: Move interfaces that affects checks to kcsan-checks.h

2020-03-21T08:43:47+00:00

This moves functions that affect state changing the behaviour of
kcsan_check_access() to kcsan-checks.h. Since these are likely used with
kcsan_check_access() it makes more sense to have them in kcsan-checks.h,
to avoid including all of 'include/linux/kcsan.h'.

No functional change intended.

Acked-by: John Hubbard 
Signed-off-by: Marco Elver 
Signed-off-by: Paul E. McKenney 
Signed-off-by: Ingo Molnar