Re: [PATCH v4 00/10] Add Kernel Concurrency Sanitizer (KCSAN)

[Paul E. McKenney]

On Thu, Nov 14, 2019 at 07:02:53PM +0100, Marco Elver wrote:
> This is the patch-series for the Kernel Concurrency Sanitizer (KCSAN).
> KCSAN is a sampling watchpoint-based *data race detector*. More details
> are included in **Documentation/dev-tools/kcsan.rst**. This patch-series
> only enables KCSAN for x86, but we expect adding support for other
> architectures is relatively straightforward (we are aware of
> experimental ARM64 and POWER support).
> 
> To gather early feedback, we announced KCSAN back in September, and have
> integrated the feedback where possible:
> http://lkml.kernel.org/r/CANpmjNPJ_bHjfLZCAPV23AXFfiPiyXXqqu72n6TgWzb2Gnu1eA@xxxxxxxxxxxxxx
> 
> The current list of known upstream fixes for data races found by KCSAN
> can be found here:
> https://github.com/google/ktsan/wiki/KCSAN#upstream-fixes-of-data-races-found-by-kcsan
> 
> We want to point out and acknowledge the work surrounding the LKMM,
> including several articles that motivate why data races are dangerous
> [1, 2], justifying a data race detector such as KCSAN.
> 
> [1] https://lwn.net/Articles/793253/
> [2] https://lwn.net/Articles/799218/

I queued this and ran a quick rcutorture on it, which completed
successfully with quite a few reports.

							Thanx, Paul

> Race conditions vs. data races
> ------------------------------
> 
> Race conditions are logic bugs, where unexpected interleaving of racing
> concurrent operations result in an erroneous state.
> 
> Data races on the other hand are defined at the *memory model/language
> level*.  Many data races are also harmful race conditions, which a tool
> like KCSAN reports!  However, not all data races are race conditions and
> vice-versa.  KCSAN's intent is to report data races according to the
> LKMM. A data race detector can only work at the memory model/language
> level.
> 
> Deeper analysis, to find high-level race conditions only, requires
> conveying the intended kernel logic to a tool. This requires (1) the
> developer writing a specification or model of their code, and then (2)
> the tool verifying that the implementation matches. This has been done
> for small bits of code using model checkers and other formal methods,
> but does not scale to the level of what can be covered with a dynamic
> analysis based data race detector such as KCSAN.
> 
> For reasons outlined in [1, 2], data races can be much more subtle, but
> can cause no less harm than high-level race conditions.
> 
> Changelog
> ---------
> v4:
> * Major changes:
>  - Optimizations resulting in performance improvement of 33% (on
>    microbenchmark).
>  - Deal with nested interrupts for atomic_next.
>  - Simplify report.c (removing double-locking as well), in preparation
>    for KCSAN_REPORT_VALUE_CHANGE_ONLY.
>  - Add patch to introduce "data_race(expr)" macro.
>  - Introduce KCSAN_REPORT_VALUE_CHANGE_ONLY option for further filtering of data
>    races: if a conflicting write was observed via a watchpoint, only report the
>    data race if a value change was observed as well. The option will be enabled
>    by default on syzbot. (rcu-functions will be excluded from this filter at
>    request of Paul McKenney.) Context:
>    http://lkml.kernel.org/r/CANpmjNOepvb6+zJmDePxj21n2rctM4Sp4rJ66x_J-L1UmNK54A@xxxxxxxxxxxxxx
> 
> v3: http://lkml.kernel.org/r/20191104142745.14722-1-elver@xxxxxxxxxx
> * Major changes:
>  - Add microbenchmark.
>  - Add instruction watchpoint skip randomization.
>  - Refactor API and core runtime fast-path and slow-path. Compared to
>    the previous version, with a default config and benchmarked using the
>    added microbenchmark, this version is 3.8x faster.
>  - Make __tsan_unaligned __alias of generic accesses.
>  - Rename kcsan_{begin,end}_atomic ->
>    kcsan_{nestable,flat}_atomic_{begin,end}
>  - For filter list in debugfs.c use kmalloc+krealloc instead of
>    kvmalloc.
>  - Split Documentation into separate patch.
> 
> v2: http://lkml.kernel.org/r/20191017141305.146193-1-elver@xxxxxxxxxx
> * Major changes:
>  - Replace kcsan_check_access(.., {true, false}) with
>    kcsan_check_{read,write}.
>  - Change atomic-instrumented.h to use __atomic_check_{read,write}.
>  - Use common struct kcsan_ctx in task_struct and for per-CPU interrupt
>    contexts.
> 
> v1: http://lkml.kernel.org/r/20191016083959.186860-1-elver@xxxxxxxxxx
> 
> Marco Elver (10):
>   kcsan: Add Kernel Concurrency Sanitizer infrastructure
>   include/linux/compiler.h: Introduce data_race(expr) macro
>   kcsan: Add Documentation entry in dev-tools
>   objtool, kcsan: Add KCSAN runtime functions to whitelist
>   build, kcsan: Add KCSAN build exceptions
>   seqlock, kcsan: Add annotations for KCSAN
>   seqlock: Require WRITE_ONCE surrounding raw_seqcount_barrier
>   asm-generic, kcsan: Add KCSAN instrumentation for bitops
>   locking/atomics, kcsan: Add KCSAN instrumentation
>   x86, kcsan: Enable KCSAN for x86
> 
>  Documentation/dev-tools/index.rst         |   1 +
>  Documentation/dev-tools/kcsan.rst         | 256 +++++++++
>  MAINTAINERS                               |  11 +
>  Makefile                                  |   3 +-
>  arch/x86/Kconfig                          |   1 +
>  arch/x86/boot/Makefile                    |   2 +
>  arch/x86/boot/compressed/Makefile         |   2 +
>  arch/x86/entry/vdso/Makefile              |   3 +
>  arch/x86/include/asm/bitops.h             |   6 +-
>  arch/x86/kernel/Makefile                  |   4 +
>  arch/x86/kernel/cpu/Makefile              |   3 +
>  arch/x86/lib/Makefile                     |   4 +
>  arch/x86/mm/Makefile                      |   4 +
>  arch/x86/purgatory/Makefile               |   2 +
>  arch/x86/realmode/Makefile                |   3 +
>  arch/x86/realmode/rm/Makefile             |   3 +
>  drivers/firmware/efi/libstub/Makefile     |   2 +
>  include/asm-generic/atomic-instrumented.h | 393 +++++++-------
>  include/asm-generic/bitops-instrumented.h |  18 +
>  include/linux/compiler-clang.h            |   9 +
>  include/linux/compiler-gcc.h              |   7 +
>  include/linux/compiler.h                  |  57 +-
>  include/linux/kcsan-checks.h              |  97 ++++
>  include/linux/kcsan.h                     | 115 ++++
>  include/linux/sched.h                     |   4 +
>  include/linux/seqlock.h                   |  51 +-
>  init/init_task.c                          |   8 +
>  init/main.c                               |   2 +
>  kernel/Makefile                           |   6 +
>  kernel/kcsan/Makefile                     |  11 +
>  kernel/kcsan/atomic.h                     |  27 +
>  kernel/kcsan/core.c                       | 626 ++++++++++++++++++++++
>  kernel/kcsan/debugfs.c                    | 275 ++++++++++
>  kernel/kcsan/encoding.h                   |  94 ++++
>  kernel/kcsan/kcsan.h                      | 108 ++++
>  kernel/kcsan/report.c                     | 320 +++++++++++
>  kernel/kcsan/test.c                       | 121 +++++
>  kernel/sched/Makefile                     |   6 +
>  lib/Kconfig.debug                         |   2 +
>  lib/Kconfig.kcsan                         | 118 ++++
>  lib/Makefile                              |   3 +
>  mm/Makefile                               |   8 +
>  scripts/Makefile.kcsan                    |   6 +
>  scripts/Makefile.lib                      |  10 +
>  scripts/atomic/gen-atomic-instrumented.sh |  17 +-
>  tools/objtool/check.c                     |  18 +
>  46 files changed, 2641 insertions(+), 206 deletions(-)
>  create mode 100644 Documentation/dev-tools/kcsan.rst
>  create mode 100644 include/linux/kcsan-checks.h
>  create mode 100644 include/linux/kcsan.h
>  create mode 100644 kernel/kcsan/Makefile
>  create mode 100644 kernel/kcsan/atomic.h
>  create mode 100644 kernel/kcsan/core.c
>  create mode 100644 kernel/kcsan/debugfs.c
>  create mode 100644 kernel/kcsan/encoding.h
>  create mode 100644 kernel/kcsan/kcsan.h
>  create mode 100644 kernel/kcsan/report.c
>  create mode 100644 kernel/kcsan/test.c
>  create mode 100644 lib/Kconfig.kcsan
>  create mode 100644 scripts/Makefile.kcsan
> 
> -- 
> 2.24.0.rc1.363.gb1bccd3e3d-goog
>