Documentation/dev-tools/kcsan.rst

   1 .. SPDX-License-Identifier: GPL-2.0
   2 .. Copyright (C) 2019, Google LLC.
   3
   4 The Kernel Concurrency Sanitizer (KCSAN)
   5 ========================================
   6
   7 The Kernel Concurrency Sanitizer (KCSAN) is a dynamic race detector, which
   8 relies on compile-time instrumentation, and uses a watchpoint-based sampling
   9 approach to detect races. KCSAN's primary purpose is to detect `data races`_.
  10
  11 Usage
  12 -----
  13
  14 KCSAN is supported by both GCC and Clang. With GCC we require version 11 or
  15 later, and with Clang also require version 11 or later.
  16
  17 To enable KCSAN configure the kernel with::
  18
  19     CONFIG_KCSAN = y
  20
  21 KCSAN provides several other configuration options to customize behaviour (see
  22 the respective help text in ``lib/Kconfig.kcsan`` for more info).
  23
  24 Error reports
  25 ~~~~~~~~~~~~~
  26
  27 A typical data race report looks like this::
  28
  29     ==================================================================
  30     BUG: KCSAN: data-race in test_kernel_read / test_kernel_write
  31
  32     write to 0xffffffffc009a628 of 8 bytes by task 487 on cpu 0:
  33      test_kernel_write+0x1d/0x30
  34      access_thread+0x89/0xd0
  35      kthread+0x23e/0x260
  36      ret_from_fork+0x22/0x30
  37
  38     read to 0xffffffffc009a628 of 8 bytes by task 488 on cpu 6:
  39      test_kernel_read+0x10/0x20
  40      access_thread+0x89/0xd0
  41      kthread+0x23e/0x260
  42      ret_from_fork+0x22/0x30
  43
  44     value changed: 0x00000000000009a6 -> 0x00000000000009b2
  45
  46     Reported by Kernel Concurrency Sanitizer on:
  47     CPU: 6 PID: 488 Comm: access_thread Not tainted 5.12.0-rc2+ #1
  48     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
  49     ==================================================================
  50
  51 The header of the report provides a short summary of the functions involved in
  52 the race. It is followed by the access types and stack traces of the 2 threads
  53 involved in the data race. If KCSAN also observed a value change, the observed
  54 old value and new value are shown on the "value changed" line respectively.
  55
  56 The other less common type of data race report looks like this::
  57
  58     ==================================================================
  59     BUG: KCSAN: data-race in test_kernel_rmw_array+0x71/0xd0
  60
  61     race at unknown origin, with read to 0xffffffffc009bdb0 of 8 bytes by task 515 on cpu 2:
  62      test_kernel_rmw_array+0x71/0xd0
  63      access_thread+0x89/0xd0
  64      kthread+0x23e/0x260
  65      ret_from_fork+0x22/0x30
  66
  67     value changed: 0x0000000000002328 -> 0x0000000000002329
  68
  69     Reported by Kernel Concurrency Sanitizer on:
  70     CPU: 2 PID: 515 Comm: access_thread Not tainted 5.12.0-rc2+ #1
  71     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014
  72     ==================================================================
  73
  74 This report is generated where it was not possible to determine the other
  75 racing thread, but a race was inferred due to the data value of the watched
  76 memory location having changed. These reports always show a "value changed"
  77 line. A common reason for reports of this type are missing instrumentation in
  78 the racing thread, but could also occur due to e.g. DMA accesses. Such reports
  79 are shown only if ``CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=y``, which is
  80 enabled by default.
  81
  82 Selective analysis
  83 ~~~~~~~~~~~~~~~~~~
  84
  85 It may be desirable to disable data race detection for specific accesses,
  86 functions, compilation units, or entire subsystems.  For static blacklisting,
  87 the below options are available:
  88
  89 * KCSAN understands the ``data_race(expr)`` annotation, which tells KCSAN that
  90   any data races due to accesses in ``expr`` should be ignored and resulting
  91   behaviour when encountering a data race is deemed safe.  Please see
  92   `"Marking Shared-Memory Accesses" in the LKMM`_ for more information.
  93
  94 * Disabling data race detection for entire functions can be accomplished by
  95   using the function attribute ``__no_kcsan``::
  96
  97     __no_kcsan
  98     void foo(void) {
  99         ...
 100
 101   To dynamically limit for which functions to generate reports, see the
 102   `DebugFS interface`_ blacklist/whitelist feature.
 103
 104 * To disable data race detection for a particular compilation unit, add to the
 105   ``Makefile``::
 106
 107     KCSAN_SANITIZE_file.o := n
 108
 109 * To disable data race detection for all compilation units listed in a
 110   ``Makefile``, add to the respective ``Makefile``::
 111
 112     KCSAN_SANITIZE := n
 113
 114 .. _"Marking Shared-Memory Accesses" in the LKMM: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt
 115
 116 Furthermore, it is possible to tell KCSAN to show or hide entire classes of
 117 data races, depending on preferences. These can be changed via the following
 118 Kconfig options:
 119
 120 * ``CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY``: If enabled and a conflicting write
 121   is observed via a watchpoint, but the data value of the memory location was
 122   observed to remain unchanged, do not report the data race.
 123
 124 * ``CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC``: Assume that plain aligned writes
 125   up to word size are atomic by default. Assumes that such writes are not
 126   subject to unsafe compiler optimizations resulting in data races. The option
 127   causes KCSAN to not report data races due to conflicts where the only plain
 128   accesses are aligned writes up to word size.
 129
 130 * ``CONFIG_KCSAN_PERMISSIVE``: Enable additional permissive rules to ignore
 131   certain classes of common data races. Unlike the above, the rules are more
 132   complex involving value-change patterns, access type, and address. This
 133   option depends on ``CONFIG_KCSAN_REPORT_VALUE_CHANGE_ONLY=y``. For details
 134   please see the ``kernel/kcsan/permissive.h``. Testers and maintainers that
 135   only focus on reports from specific subsystems and not the whole kernel are
 136   recommended to disable this option.
 137
 138 To use the strictest possible rules, select ``CONFIG_KCSAN_STRICT=y``, which
 139 configures KCSAN to follow the Linux-kernel memory consistency model (LKMM) as
 140 closely as possible.
 141
 142 DebugFS interface
 143 ~~~~~~~~~~~~~~~~~
 144
 145 The file ``/sys/kernel/debug/kcsan`` provides the following interface:
 146
 147 * Reading ``/sys/kernel/debug/kcsan`` returns various runtime statistics.
 148
 149 * Writing ``on`` or ``off`` to ``/sys/kernel/debug/kcsan`` allows turning KCSAN
 150   on or off, respectively.
 151
 152 * Writing ``!some_func_name`` to ``/sys/kernel/debug/kcsan`` adds
 153   ``some_func_name`` to the report filter list, which (by default) blacklists
 154   reporting data races where either one of the top stackframes are a function
 155   in the list.
 156
 157 * Writing either ``blacklist`` or ``whitelist`` to ``/sys/kernel/debug/kcsan``
 158   changes the report filtering behaviour. For example, the blacklist feature
 159   can be used to silence frequently occurring data races; the whitelist feature
 160   can help with reproduction and testing of fixes.
 161
 162 Tuning performance
 163 ~~~~~~~~~~~~~~~~~~
 164
 165 Core parameters that affect KCSAN's overall performance and bug detection
 166 ability are exposed as kernel command-line arguments whose defaults can also be
 167 changed via the corresponding Kconfig options.
 168
 169 * ``kcsan.skip_watch`` (``CONFIG_KCSAN_SKIP_WATCH``): Number of per-CPU memory
 170   operations to skip, before another watchpoint is set up. Setting up
 171   watchpoints more frequently will result in the likelihood of races to be
 172   observed to increase. This parameter has the most significant impact on
 173   overall system performance and race detection ability.
 174
 175 * ``kcsan.udelay_task`` (``CONFIG_KCSAN_UDELAY_TASK``): For tasks, the
 176   microsecond delay to stall execution after a watchpoint has been set up.
 177   Larger values result in the window in which we may observe a race to
 178   increase.
 179
 180 * ``kcsan.udelay_interrupt`` (``CONFIG_KCSAN_UDELAY_INTERRUPT``): For
 181   interrupts, the microsecond delay to stall execution after a watchpoint has
 182   been set up. Interrupts have tighter latency requirements, and their delay
 183   should generally be smaller than the one chosen for tasks.
 184
 185 They may be tweaked at runtime via ``/sys/module/kcsan/parameters/``.
 186
 187 Data Races
 188 ----------
 189
 190 In an execution, two memory accesses form a *data race* if they *conflict*,
 191 they happen concurrently in different threads, and at least one of them is a
 192 *plain access*; they *conflict* if both access the same memory location, and at
 193 least one is a write. For a more thorough discussion and definition, see `"Plain
 194 Accesses and Data Races" in the LKMM`_.
 195
 196 .. _"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
 197
 198 Relationship with the Linux-Kernel Memory Consistency Model (LKMM)
 199 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 200
 201 The LKMM defines the propagation and ordering rules of various memory
 202 operations, which gives developers the ability to reason about concurrent code.
 203 Ultimately this allows to determine the possible executions of concurrent code,
 204 and if that code is free from data races.
 205
 206 KCSAN is aware of *marked atomic operations* (``READ_ONCE``, ``WRITE_ONCE``,
 207 ``atomic_*``, etc.), and a subset of ordering guarantees implied by memory
 208 barriers. With ``CONFIG_KCSAN_WEAK_MEMORY=y``, KCSAN models load or store
 209 buffering, and can detect missing ``smp_mb()``, ``smp_wmb()``, ``smp_rmb()``,
 210 ``smp_store_release()``, and all ``atomic_*`` operations with equivalent
 211 implied barriers.
 212
 213 Note, KCSAN will not report all data races due to missing memory ordering,
 214 specifically where a memory barrier would be required to prohibit subsequent
 215 memory operation from reordering before the barrier. Developers should
 216 therefore carefully consider the required memory ordering requirements that
 217 remain unchecked.
 218
 219 Race Detection Beyond Data Races
 220 --------------------------------
 221
 222 For code with complex concurrency design, race-condition bugs may not always
 223 manifest as data races. Race conditions occur if concurrently executing
 224 operations result in unexpected system behaviour. On the other hand, data races
 225 are defined at the C-language level. The following macros can be used to check
 226 properties of concurrent code where bugs would not manifest as data races.
 227
 228 .. kernel-doc:: include/linux/kcsan-checks.h
 229     :functions: ASSERT_EXCLUSIVE_WRITER ASSERT_EXCLUSIVE_WRITER_SCOPED
 230                 ASSERT_EXCLUSIVE_ACCESS ASSERT_EXCLUSIVE_ACCESS_SCOPED
 231                 ASSERT_EXCLUSIVE_BITS
 232
 233 Implementation Details
 234 ----------------------
 235
 236 KCSAN relies on observing that two accesses happen concurrently. Crucially, we
 237 want to (a) increase the chances of observing races (especially for races that
 238 manifest rarely), and (b) be able to actually observe them. We can accomplish
 239 (a) by injecting various delays, and (b) by using address watchpoints (or
 240 breakpoints).
 241
 242 If we deliberately stall a memory access, while we have a watchpoint for its
 243 address set up, and then observe the watchpoint to fire, two accesses to the
 244 same address just raced. Using hardware watchpoints, this is the approach taken
 245 in `DataCollider
 246 <http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf>`_.
 247 Unlike DataCollider, KCSAN does not use hardware watchpoints, but instead
 248 relies on compiler instrumentation and "soft watchpoints".
 249
 250 In KCSAN, watchpoints are implemented using an efficient encoding that stores
 251 access type, size, and address in a long; the benefits of using "soft
 252 watchpoints" are portability and greater flexibility. KCSAN then relies on the
 253 compiler instrumenting plain accesses. For each instrumented plain access:
 254
 255 1. Check if a matching watchpoint exists; if yes, and at least one access is a
 256    write, then we encountered a racing access.
 257
 258 2. Periodically, if no matching watchpoint exists, set up a watchpoint and
 259    stall for a small randomized delay.
 260
 261 3. Also check the data value before the delay, and re-check the data value
 262    after delay; if the values mismatch, we infer a race of unknown origin.
 263
 264 To detect data races between plain and marked accesses, KCSAN also annotates
 265 marked accesses, but only to check if a watchpoint exists; i.e. KCSAN never
 266 sets up a watchpoint on marked accesses. By never setting up watchpoints for
 267 marked operations, if all accesses to a variable that is accessed concurrently
 268 are properly marked, KCSAN will never trigger a watchpoint and therefore never
 269 report the accesses.
 270
 271 Modeling Weak Memory
 272 ~~~~~~~~~~~~~~~~~~~~
 273
 274 KCSAN's approach to detecting data races due to missing memory barriers is
 275 based on modeling access reordering (with ``CONFIG_KCSAN_WEAK_MEMORY=y``).
 276 Each plain memory access for which a watchpoint is set up, is also selected for
 277 simulated reordering within the scope of its function (at most 1 in-flight
 278 access).
 279
 280 Once an access has been selected for reordering, it is checked along every
 281 other access until the end of the function scope. If an appropriate memory
 282 barrier is encountered, the access will no longer be considered for simulated
 283 reordering.
 284
 285 When the result of a memory operation should be ordered by a barrier, KCSAN can
 286 then detect data races where the conflict only occurs as a result of a missing
 287 barrier. Consider the example::
 288
 289     int x, flag;
 290     void T1(void)
 291     {
 292         x = 1;                  // data race!
 293         WRITE_ONCE(flag, 1);    // correct: smp_store_release(&flag, 1)
 294     }
 295     void T2(void)
 296     {
 297         while (!READ_ONCE(flag));   // correct: smp_load_acquire(&flag)
 298         ... = x;                    // data race!
 299     }
 300
 301 When weak memory modeling is enabled, KCSAN can consider ``x`` in ``T1`` for
 302 simulated reordering. After the write of ``flag``, ``x`` is again checked for
 303 concurrent accesses: because ``T2`` is able to proceed after the write of
 304 ``flag``, a data race is detected. With the correct barriers in place, ``x``
 305 would not be considered for reordering after the proper release of ``flag``,
 306 and no data race would be detected.
 307
 308 Deliberate trade-offs in complexity but also practical limitations mean only a
 309 subset of data races due to missing memory barriers can be detected. With
 310 currently available compiler support, the implementation is limited to modeling
 311 the effects of "buffering" (delaying accesses), since the runtime cannot
 312 "prefetch" accesses. Also recall that watchpoints are only set up for plain
 313 accesses, and the only access type for which KCSAN simulates reordering. This
 314 means reordering of marked accesses is not modeled.
 315
 316 A consequence of the above is that acquire operations do not require barrier
 317 instrumentation (no prefetching). Furthermore, marked accesses introducing
 318 address or control dependencies do not require special handling (the marked
 319 access cannot be reordered, later dependent accesses cannot be prefetched).
 320
 321 Key Properties
 322 ~~~~~~~~~~~~~~
 323
 324 1. **Memory Overhead:**  The overall memory overhead is only a few MiB
 325    depending on configuration. The current implementation uses a small array of
 326    longs to encode watchpoint information, which is negligible.
 327
 328 2. **Performance Overhead:** KCSAN's runtime aims to be minimal, using an
 329    efficient watchpoint encoding that does not require acquiring any shared
 330    locks in the fast-path. For kernel boot on a system with 8 CPUs:
 331
 332    - 5.0x slow-down with the default KCSAN config;
 333    - 2.8x slow-down from runtime fast-path overhead only (set very large
 334      ``KCSAN_SKIP_WATCH`` and unset ``KCSAN_SKIP_WATCH_RANDOMIZE``).
 335
 336 3. **Annotation Overheads:** Minimal annotations are required outside the KCSAN
 337    runtime. As a result, maintenance overheads are minimal as the kernel
 338    evolves.
 339
 340 4. **Detects Racy Writes from Devices:** Due to checking data values upon
 341    setting up watchpoints, racy writes from devices can also be detected.
 342
 343 5. **Memory Ordering:** KCSAN is aware of only a subset of LKMM ordering rules;
 344    this may result in missed data races (false negatives).
 345
 346 6. **Analysis Accuracy:** For observed executions, due to using a sampling
 347    strategy, the analysis is *unsound* (false negatives possible), but aims to
 348    be complete (no false positives).
 349
 350 Alternatives Considered
 351 -----------------------
 352
 353 An alternative data race detection approach for the kernel can be found in the
 354 `Kernel Thread Sanitizer (KTSAN) <https://github.com/google/ktsan/wiki>`_.
 355 KTSAN is a happens-before data race detector, which explicitly establishes the
 356 happens-before order between memory operations, which can then be used to
 357 determine data races as defined in `Data Races`_.
 358
 359 To build a correct happens-before relation, KTSAN must be aware of all ordering
 360 rules of the LKMM and synchronization primitives. Unfortunately, any omission
 361 leads to large numbers of false positives, which is especially detrimental in
 362 the context of the kernel which includes numerous custom synchronization
 363 mechanisms. To track the happens-before relation, KTSAN's implementation
 364 requires metadata for each memory location (shadow memory), which for each page
 365 corresponds to 4 pages of shadow memory, and can translate into overhead of
 366 tens of GiB on a large system.