The Kernel Concurrency Sanitizer (KCSAN) ======================================== The Kernel Concurrency Sanitizer (KCSAN) is a dynamic race detector, which relies on compile-time instrumentation, and uses a watchpoint-based sampling approach to detect races. KCSAN's primary purpose is to detect `data races`_. Usage ----- KCSAN is supported by both GCC and Clang. With GCC we require version 11 or later, and with Clang also require version 11 or later. To enable KCSAN configure the kernel with:: CONFIG_KCSAN = y KCSAN provides several other configuration options to customize behaviour (see the respective help text in ``lib/Kconfig.kcsan`` for more info). Error reports ~~~~~~~~~~~~~ A typical data race report looks like this:: ================================================================== BUG: KCSAN: data-race in generic_permission / kernfs_refresh_inode write to 0xffff8fee4c40700c of 4 bytes by task 175 on cpu 4: kernfs_refresh_inode+0x70/0x170 kernfs_iop_permission+0x4f/0x90 inode_permission+0x190/0x200 link_path_walk.part.0+0x503/0x8e0 path_lookupat.isra.0+0x69/0x4d0 filename_lookup+0x136/0x280 user_path_at_empty+0x47/0x60 vfs_statx+0x9b/0x130 __do_sys_newlstat+0x50/0xb0 __x64_sys_newlstat+0x37/0x50 do_syscall_64+0x85/0x260 entry_SYSCALL_64_after_hwframe+0x44/0xa9 read to 0xffff8fee4c40700c of 4 bytes by task 166 on cpu 6: generic_permission+0x5b/0x2a0 kernfs_iop_permission+0x66/0x90 inode_permission+0x190/0x200 link_path_walk.part.0+0x503/0x8e0 path_lookupat.isra.0+0x69/0x4d0 filename_lookup+0x136/0x280 user_path_at_empty+0x47/0x60 do_faccessat+0x11a/0x390 __x64_sys_access+0x3c/0x50 do_syscall_64+0x85/0x260 entry_SYSCALL_64_after_hwframe+0x44/0xa9 Reported by Kernel Concurrency Sanitizer on: CPU: 6 PID: 166 Comm: systemd-journal Not tainted 5.3.0-rc7+ #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 ================================================================== The header of the report provides a short summary of the functions involved in the race. It is followed by the access types and stack traces of the 2 threads involved in the data race. The other less common type of data race report looks like this:: ================================================================== BUG: KCSAN: data-race in e1000_clean_rx_irq+0x551/0xb10 race at unknown origin, with read to 0xffff933db8a2ae6c of 1 bytes by interrupt on cpu 0: e1000_clean_rx_irq+0x551/0xb10 e1000_clean+0x533/0xda0 net_rx_action+0x329/0x900 __do_softirq+0xdb/0x2db irq_exit+0x9b/0xa0 do_IRQ+0x9c/0xf0 ret_from_intr+0x0/0x18 default_idle+0x3f/0x220 arch_cpu_idle+0x21/0x30 do_idle+0x1df/0x230 cpu_startup_entry+0x14/0x20 rest_init+0xc5/0xcb arch_call_rest_init+0x13/0x2b start_kernel+0x6db/0x700 Reported by Kernel Concurrency Sanitizer on: CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.3.0-rc7+ #2 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014 ================================================================== This report is generated where it was not possible to determine the other racing thread, but a race was inferred due to the data value of the watched memory location having changed. These can occur either due to missing instrumentation or e.g. DMA accesses. These reports will only be generated if ``CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=y`` (selected by default). Selective analysis ~~~~~~~~~~~~~~~~~~ It may be desirable to disable data race detection for specific accesses, functions, compilation units, or entire subsystems. For static blacklisting, the below options are available: * KCSAN understands the ``data_race(expr)`` annotation, which tells KCSAN that any data races due to accesses in ``expr`` should be ignored and resulting behaviour when encountering a data race is deemed safe. * Disabling data race detection for entire functions can be accomplished by using the function attribute ``__no_kcsan``:: __no_kcsan void foo(void) { ... To dynamically limit for which functions to generate reports, see the `DebugFS interface`_ blacklist/whitelist feature. * To disable data race detection for a particular compilation unit, add to the ``Makefile``:: KCSAN_SANITIZE_file.o := n * To disable data race detection for all compilation units listed in a ``Makefile``, add to the respective ``Makefile``:: KCSAN_SANITIZE := n Furthermore, it is possible to tell KCSAN to show or hide entire classes of data races, depending on preferences. These can be changed via the following Kconfig options: * ``CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY``: 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``: Assume that plain aligned writes up to word size are atomic by default. Assumes that such writes are not subject to unsafe compiler optimizations resulting in data races. The option causes KCSAN to not report data races due to conflicts where the only plain accesses are aligned writes up to word size. DebugFS interface ~~~~~~~~~~~~~~~~~ The file ``/sys/kernel/debug/kcsan`` provides the following interface: * Reading ``/sys/kernel/debug/kcsan`` returns various runtime statistics. * Writing ``on`` or ``off`` to ``/sys/kernel/debug/kcsan`` allows turning KCSAN on or off, respectively. * Writing ``!some_func_name`` to ``/sys/kernel/debug/kcsan`` adds ``some_func_name`` to the report filter list, which (by default) blacklists reporting data races where either one of the top stackframes are a function in the list. * Writing either ``blacklist`` or ``whitelist`` to ``/sys/kernel/debug/kcsan`` changes the report filtering behaviour. For example, the blacklist feature can be used to silence frequently occurring data races; the whitelist feature can help with reproduction and testing of fixes. Tuning performance ~~~~~~~~~~~~~~~~~~ Core parameters that affect KCSAN's overall performance and bug detection ability are exposed as kernel command-line arguments whose defaults can also be changed via the corresponding Kconfig options. * ``kcsan.skip_watch`` (``CONFIG_KCSAN_SKIP_WATCH``): Number of per-CPU memory operations to skip, before another watchpoint is set up. Setting up watchpoints more frequently will result in the likelihood of races to be observed to increase. This parameter has the most significant impact on overall system performance and race detection ability. * ``kcsan.udelay_task`` (``CONFIG_KCSAN_UDELAY_TASK``): For tasks, the microsecond delay to stall execution after a watchpoint has been set up. Larger values result in the window in which we may observe a race to increase. * ``kcsan.udelay_interrupt`` (``CONFIG_KCSAN_UDELAY_INTERRUPT``): For interrupts, the microsecond delay to stall execution after a watchpoint has been set up. Interrupts have tighter latency requirements, and their delay should generally be smaller than the one chosen for tasks. They may be tweaked at runtime via ``/sys/module/kcsan/parameters/``. Data Races ---------- In an execution, two memory accesses form a *data race* if they *conflict*, they happen concurrently in different threads, and at least one of them is a *plain access*; they *conflict* if both access the same memory location, and at least one is a write. For a more thorough discussion and definition, see `"Plain Accesses and Data Races" in the LKMM`_. .. _"Plain Accesses and Data Races" in the LKMM: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/explanation.txt#n1922 Relationship with the Linux-Kernel Memory Consistency Model (LKMM) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The LKMM defines the propagation and ordering rules of various memory operations, which gives developers the ability to reason about concurrent code. Ultimately this allows to determine the possible executions of concurrent code, and if that code is free from data races. KCSAN is aware of *marked atomic operations* (``READ_ONCE``, ``WRITE_ONCE``, ``atomic_*``, etc.), but is oblivious of any ordering guarantees and simply assumes that memory barriers are placed correctly. In other words, KCSAN assumes that as long as a plain access is not observed to race with another conflicting access, memory operations are correctly ordered. This means that KCSAN will not report *potential* data races due to missing memory ordering. Developers should therefore carefully consider the required memory ordering requirements that remain unchecked. If, however, missing memory ordering (that is observable with a particular compiler and architecture) leads to an observable data race (e.g. entering a critical section erroneously), KCSAN would report the resulting data race. Race Detection Beyond Data Races -------------------------------- For code with complex concurrency design, race-condition bugs may not always manifest as data races. Race conditions occur if concurrently executing operations result in unexpected system behaviour. On the other hand, data races are defined at the C-language level. The following macros can be used to check properties of concurrent code where bugs would not manifest as data races. .. kernel-doc:: include/linux/kcsan-checks.h :functions: ASSERT_EXCLUSIVE_WRITER ASSERT_EXCLUSIVE_WRITER_SCOPED ASSERT_EXCLUSIVE_ACCESS ASSERT_EXCLUSIVE_ACCESS_SCOPED ASSERT_EXCLUSIVE_BITS Implementation Details ---------------------- KCSAN relies on observing that two accesses happen concurrently. Crucially, we want to (a) increase the chances of observing races (especially for races that manifest rarely), and (b) be able to actually observe them. We can accomplish (a) by injecting various delays, and (b) by using address watchpoints (or breakpoints). If we deliberately stall a memory access, while we have a watchpoint for its address set up, and then observe the watchpoint to fire, two accesses to the same address just raced. Using hardware watchpoints, this is the approach taken in `DataCollider `_. Unlike DataCollider, KCSAN does not use hardware watchpoints, but instead relies on compiler instrumentation and "soft watchpoints". In KCSAN, watchpoints are implemented using an efficient encoding that stores access type, size, and address in a long; the benefits of using "soft watchpoints" are portability and greater flexibility. KCSAN then relies on the compiler instrumenting plain accesses. For each instrumented plain access: 1. Check if a matching watchpoint exists; if yes, and at least one access is a write, then we encountered a racing access. 2. Periodically, if no matching watchpoint exists, set up a watchpoint and stall for a small randomized delay. 3. Also check the data value before the delay, and re-check the data value after delay; if the values mismatch, we infer a race of unknown origin. To detect data races between plain and marked accesses, KCSAN also annotates marked accesses, but only to check if a watchpoint exists; i.e. KCSAN never sets up a watchpoint on marked accesses. By never setting up watchpoints for marked operations, if all accesses to a variable that is accessed concurrently are properly marked, KCSAN will never trigger a watchpoint and therefore never report the accesses. Key Properties ~~~~~~~~~~~~~~ 1. **Memory Overhead:** The overall memory overhead is only a few MiB depending on configuration. The current implementation uses a small array of longs to encode watchpoint information, which is negligible. 2. **Performance Overhead:** KCSAN's runtime aims to be minimal, using an efficient watchpoint encoding that does not require acquiring any shared locks in the fast-path. For kernel boot on a system with 8 CPUs: - 5.0x slow-down with the default KCSAN config; - 2.8x slow-down from runtime fast-path overhead only (set very large ``KCSAN_SKIP_WATCH`` and unset ``KCSAN_SKIP_WATCH_RANDOMIZE``). 3. **Annotation Overheads:** Minimal annotations are required outside the KCSAN runtime. As a result, maintenance overheads are minimal as the kernel evolves. 4. **Detects Racy Writes from Devices:** Due to checking data values upon setting up watchpoints, racy writes from devices can also be detected. 5. **Memory Ordering:** KCSAN is *not* explicitly aware of the LKMM's ordering rules; this may result in missed data races (false negatives). 6. **Analysis Accuracy:** For observed executions, due to using a sampling strategy, the analysis is *unsound* (false negatives possible), but aims to be complete (no false positives). Alternatives Considered ----------------------- An alternative data race detection approach for the kernel can be found in the `Kernel Thread Sanitizer (KTSAN) `_. KTSAN is a happens-before data race detector, which explicitly establishes the happens-before order between memory operations, which can then be used to determine data races as defined in `Data Races`_. To build a correct happens-before relation, KTSAN must be aware of all ordering rules of the LKMM and synchronization primitives. Unfortunately, any omission leads to large numbers of false positives, which is especially detrimental in the context of the kernel which includes numerous custom synchronization mechanisms. To track the happens-before relation, KTSAN's implementation requires metadata for each memory location (shadow memory), which for each page corresponds to 4 pages of shadow memory, and can translate into overhead of tens of GiB on a large system.