Kconfig.kcsan

# SPDX-License-Identifier: GPL-2.0-only

config HAVE_ARCH_KCSAN
	bool

config HAVE_KCSAN_COMPILER
	def_bool (CC_IS_CLANG && $(cc-option,-fsanitize=thread -mllvm -tsan-distinguish-volatile=1)) || \
		 (CC_IS_GCC && $(cc-option,-fsanitize=thread --param tsan-distinguish-volatile=1))
	help
	  For the list of compilers that support KCSAN, please see
	  <file:Documentation/dev-tools/kcsan.rst>.

menuconfig KCSAN
	bool "KCSAN: dynamic data race detector"
	depends on HAVE_ARCH_KCSAN && HAVE_KCSAN_COMPILER
	depends on DEBUG_KERNEL && !KASAN
	select STACKTRACE
	help
	  The Kernel Concurrency Sanitizer (KCSAN) is a dynamic
	  data-race detector that relies on compile-time instrumentation.
	  KCSAN uses a watchpoint-based sampling approach to detect races.

	  While KCSAN's primary purpose is to detect data races, it
	  also provides assertions to check data access constraints.
	  These assertions can expose bugs that do not manifest as
	  data races.

	  See <file:Documentation/dev-tools/kcsan.rst> for more details.

if KCSAN

config CC_HAS_TSAN_COMPOUND_READ_BEFORE_WRITE
	def_bool (CC_IS_CLANG && $(cc-option,-fsanitize=thread -mllvm -tsan-compound-read-before-write=1)) || \
		 (CC_IS_GCC && $(cc-option,-fsanitize=thread --param tsan-compound-read-before-write=1))
	help
	  The compiler instruments plain compound read-write operations
	  differently (++, --, +=, -=, |=, &=, etc.), which allows KCSAN to
	  distinguish them from other plain accesses. This is currently
	  supported by Clang 12 or later.

config KCSAN_VERBOSE
	bool "Show verbose reports with more information about system state"
	depends on PROVE_LOCKING
	help
	  If enabled, reports show more information about the system state that
	  may help better analyze and debug races. This includes held locks and
	  IRQ trace events.

	  While this option should generally be benign, we call into more
	  external functions on report generation; if a race report is
	  generated from any one of them, system stability may suffer due to
	  deadlocks or recursion.  If in doubt, say N.

config KCSAN_SELFTEST
	bool "Perform short selftests on boot"
	default y
	help
	  Run KCSAN selftests on boot. On test failure, causes the kernel to
	  panic. Recommended to be enabled, ensuring critical functionality
	  works as intended.

config KCSAN_KUNIT_TEST
	tristate "KCSAN test for integrated runtime behaviour" if !KUNIT_ALL_TESTS
	default KUNIT_ALL_TESTS
	depends on TRACEPOINTS && KUNIT
	select TORTURE_TEST
	help
	  KCSAN test focusing on behaviour of the integrated runtime. Tests
	  various race scenarios, and verifies the reports generated to
	  console. Makes use of KUnit for test organization, and the Torture
	  framework for test thread control.

	  Each test case may run at least up to KCSAN_REPORT_ONCE_IN_MS
	  milliseconds. Test run duration may be optimized by building the
	  kernel and KCSAN test with KCSAN_REPORT_ONCE_IN_MS set to a lower
	  than default value.

	  Say Y here if you want the test to be built into the kernel and run
	  during boot; say M if you want the test to build as a module; say N
	  if you are unsure.

config KCSAN_EARLY_ENABLE
	bool "Early enable during boot"
	default y
	help
	  If KCSAN should be enabled globally as soon as possible. KCSAN can
	  later be enabled/disabled via debugfs.

config KCSAN_NUM_WATCHPOINTS
	int "Number of available watchpoints"
	default 64
	help
	  Total number of available watchpoints. An address range maps into a
	  specific watchpoint slot as specified in kernel/kcsan/encoding.h.
	  Although larger number of watchpoints may not be usable due to
	  limited number of CPUs, a larger value helps to improve performance
	  due to reducing cache-line contention. The chosen default is a
	  conservative value; we should almost never observe "no_capacity"
	  events (see /sys/kernel/debug/kcsan).

config KCSAN_UDELAY_TASK
	int "Delay in microseconds (for tasks)"
	default 80
	help
	  For tasks, the microsecond delay after setting up a watchpoint.

config KCSAN_UDELAY_INTERRUPT
	int "Delay in microseconds (for interrupts)"
	default 20
	help
	  For interrupts, the microsecond delay after setting up a watchpoint.
	  Interrupts have tighter latency requirements, and their delay should
	  be lower than for tasks.

config KCSAN_DELAY_RANDOMIZE
	bool "Randomize above delays"
	default y
	help
	  If delays should be randomized, where the maximum is KCSAN_UDELAY_*.
	  If false, the chosen delays are always the KCSAN_UDELAY_* values
	  as defined above.

config KCSAN_SKIP_WATCH
	int "Skip instructions before setting up watchpoint"
	default 4000
	help
	  The number of per-CPU memory operations to skip, before another
	  watchpoint is set up, i.e. one in KCSAN_WATCH_SKIP per-CPU
	  memory operations are used to set up a watchpoint. A smaller value
	  results in more aggressive race detection, whereas a larger value
	  improves system performance at the cost of missing some races.

config KCSAN_SKIP_WATCH_RANDOMIZE
	bool "Randomize watchpoint instruction skip count"
	default y
	help
	  If instruction skip count should be randomized, where the maximum is
	  KCSAN_WATCH_SKIP. If false, the chosen value is always
	  KCSAN_WATCH_SKIP.

config KCSAN_INTERRUPT_WATCHER
	bool "Interruptible watchers" if !KCSAN_STRICT
	default KCSAN_STRICT
	help
	  If enabled, a task that set up a watchpoint may be interrupted while
	  delayed. This option will allow KCSAN to detect races between
	  interrupted tasks and other threads of execution on the same CPU.

	  Currently disabled by default, because not all safe per-CPU access
	  primitives and patterns may be accounted for, and therefore could
	  result in false positives.

config KCSAN_REPORT_ONCE_IN_MS
	int "Duration in milliseconds, in which any given race is only reported once"
	default 3000
	help
	  Any given race is only reported once in the defined time window.
	  Different races may still generate reports within a duration that is
	  smaller than the duration defined here. This allows rate limiting
	  reporting to avoid flooding the console with reports.  Setting this
	  to 0 disables rate limiting.

# The main purpose of the below options is to control reported data races, and
# are not expected to be switched frequently by non-testers or at runtime.
# The defaults are chosen to be conservative, and can miss certain bugs.

config KCSAN_REPORT_RACE_UNKNOWN_ORIGIN
	bool "Report races of unknown origin"
	default y
	help
	  If KCSAN should report races where only one access is known, and the
	  conflicting access is of unknown origin. This type of race is
	  reported if it was only possible to infer a race due to a data value
	  change while an access is being delayed on a watchpoint.

config KCSAN_STRICT
	bool "Strict data-race checking"
	help
	  KCSAN will report data races with the strictest possible rules, which
	  closely aligns with the rules defined by the Linux-kernel memory
	  consistency model (LKMM).

config KCSAN_WEAK_MEMORY
	bool "Enable weak memory modeling to detect missing memory barriers"
	default y
	depends on KCSAN_STRICT
	# We can either let objtool nop __tsan_func_{entry,exit}() and builtin
	# atomics instrumentation in .noinstr.text, or use a compiler that can
	# implement __no_kcsan to really remove all instrumentation.
	depends on !ARCH_WANTS_NO_INSTR || HAVE_NOINSTR_HACK || \
		   CC_IS_GCC || CLANG_VERSION >= 140000
	select OBJTOOL if HAVE_NOINSTR_HACK
	help
	  Enable support for modeling a subset of weak memory, which allows
	  detecting a subset of data races due to missing memory barriers.

	  Depends on KCSAN_STRICT, because the options strenghtening certain
	  plain accesses by default (depending on !KCSAN_STRICT) reduce the
	  ability to detect any data races invoving reordered accesses, in
	  particular reordered writes.

	  Weak memory modeling relies on additional instrumentation and may
	  affect performance.

config KCSAN_REPORT_VALUE_CHANGE_ONLY
	bool "Only report races where watcher observed a data value change"
	default y
	depends on !KCSAN_STRICT
	help
	  If enabled and a conflicting write is observed via a watchpoint, but
	  the data value of the memory location was observed to remain
	  unchanged, do not report the data race.

config KCSAN_ASSUME_PLAIN_WRITES_ATOMIC
	bool "Assume that plain aligned writes up to word size are atomic"
	default y
	depends on !KCSAN_STRICT
	help
	  Assume that plain aligned writes up to word size are atomic by
	  default, and also not subject to other unsafe compiler optimizations
	  resulting in data races. This will cause KCSAN to not report data
	  races due to conflicts where the only plain accesses are aligned
	  writes up to word size: conflicts between marked reads and plain
	  aligned writes up to word size will not be reported as data races;
	  notice that data races between two conflicting plain aligned writes
	  will also not be reported.

config KCSAN_IGNORE_ATOMICS
	bool "Do not instrument marked atomic accesses"
	depends on !KCSAN_STRICT
	help
	  Never instrument marked atomic accesses. This option can be used for
	  additional filtering. Conflicting marked atomic reads and plain
	  writes will never be reported as a data race, however, will cause
	  plain reads and marked writes to result in "unknown origin" reports.
	  If combined with CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=n, data
	  races where at least one access is marked atomic will never be
	  reported.

	  Similar to KCSAN_ASSUME_PLAIN_WRITES_ATOMIC, but including unaligned
	  accesses, conflicting marked atomic reads and plain writes will not
	  be reported as data races; however, unlike that option, data races
	  due to two conflicting plain writes will be reported (aligned and
	  unaligned, if CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=n).

config KCSAN_PERMISSIVE
	bool "Enable all additional permissive rules"
	depends on KCSAN_REPORT_VALUE_CHANGE_ONLY
	help
	  Enable additional permissive rules to ignore certain classes of data
	  races (also see kernel/kcsan/permissive.h). None of the permissive
	  rules imply that such data races are generally safe, but can be used
	  to further reduce reported data races due to data-racy patterns
	  common across the kernel.

endif # KCSAN