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KCOV Remote uses two separate memory buffers, one private to the kernel space (kcov_remote_areas) and the second one shared between user and kernel space (kcov->area). After every pair of kcov_remote_start() and kcov_remote_stop(), the coverage data collected in the kcov_remote_areas is copied to kcov->area so the user can read the collected coverage data. This memcpy() is located in kcov_move_area(). The load/store pattern on the kernel-side [1] is: ``` /* dst_area === kcov->area, dst_area[0] is where the count is stored */ dst_len = READ_ONCE(*(unsigned long *)dst_area); ... memcpy(dst_entries, src_entries, ...); ... WRITE_ONCE(*(unsigned long *)dst_area, dst_len + entries_moved); ``` And for the user [2]: ``` /* cover is equivalent to kcov->area */ n = __atomic_load_n(&cover[0], __ATOMIC_RELAXED); ``` Without a write-memory barrier, the atomic load for the user can potentially read fresh values of the count stored at cover[0], but continue to read stale coverage data from the buffer itself. Hence, we recommend adding a write-memory barrier between the memcpy() and the WRITE_ONCE() in kcov_move_area(). Link: https://lkml.kernel.org/r/20250728184318.1839137-1-soham.bagchi@utah.edu Link: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/kcov.c?h=master#n978 [1] Link: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/Documentation/dev-tools/kcov.rst#n364 [2] Signed-off-by: Soham Bagchi <soham.bagchi@utah.edu> Reviewed-by: Marco Elver <elver@google.com> Cc: Alexander Potapenko <glider@google.com> Cc: Andrey Konovalov <andreyknvl@gmail.com> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Dmitriy Vyukov <dvyukov@google.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Thomas Gleinxer <tglx@linutronix.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.