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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
<http://usenix.org/legacy/events/osdi10/tech/full_papers/Erickson.pdf>`_.
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) <https://github.com/google/ktsan/wiki>`_.
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.