Merge tag 'printk-for-6.18' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

- Add KUnit test for the printk ring buffer

- Fix the check of the

Merge tag 'printk-for-6.18' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

- Add KUnit test for the printk ring buffer

- Fix the check of the maximal record size which is allowed to be
stored into the printk ring buffer. It prevents corruptions of the
ring buffer.

Note that printk() is on the safe side. The messages are limited by
1kB buffer and are always small enough for the minimal log buffer
size 4kB, see CONFIG_LOG_BUF_SHIFT definition.

* tag 'printk-for-6.18' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux:
printk: ringbuffer: Fix data block max size check
printk: kunit: support offstack cpumask
printk: kunit: Fix __counted_by() in struct prbtest_rbdata
printk: ringbuffer: Explain why the KUnit test ignores failed writes
printk: ringbuffer: Add KUnit test

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Merge branch 'rework/ringbuffer-kunit-test' into for-linus

printk: kunit: support offstack cpumask

For large values of CONFIG_NR_CPUS, the newly added kunit test fails
to build:

kernel/printk/printk_ringbuffer_kunit_test.c: In function 'test_readerwriter':

printk: kunit: support offstack cpumask

For large values of CONFIG_NR_CPUS, the newly added kunit test fails
to build:

kernel/printk/printk_ringbuffer_kunit_test.c: In function 'test_readerwriter':
kernel/printk/printk_ringbuffer_kunit_test.c:279:1: error: the frame size of 1432 bytes is larger than 1280 bytes [-Werror=frame-larger-than=]

Change this to use cpumask_var_t and allocate it dynamically when
CONFIG_CPUMASK_OFFSTACK is set.

The variable has to be released via a KUnit action wrapper so that it is
freed when the test fails and gets aborted. The parameter type is hardcoded
to "struct cpumask *" because the macro KUNIT_DEFINE_ACTION_WRAPPER()
does not accept an array. But the function does nothing when
CONFIG_CPUMASK_OFFSTACK is not set anyway.

Fixes: 5ea2bcdfbf46 ("printk: ringbuffer: Add KUnit test")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/all/20250620192554.2234184-1-arnd@kernel.org # v1
[pmladek@suse.com: Correctly handle allocation failures and freeing using KUnit test API.]
Link: https://lore.kernel.org/all/20250702095157.110916-3-pmladek@suse.com # v2
Signed-off-by: Petr Mladek <pmladek@suse.com>

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printk: kunit: Fix __counted_by() in struct prbtest_rbdata

__counted_by() has to point to a variable which defines the size
of the related array. The code must never access the array
beyond this lim

printk: kunit: Fix __counted_by() in struct prbtest_rbdata

__counted_by() has to point to a variable which defines the size
of the related array. The code must never access the array
beyond this limit.

struct prbtest_rbdata currently stores the length of the string.
And the code access the array beyond the limit when writing
or reading the trailing '\0'.

Store the size of the string, including the trailing '\0' if
we wanted to keep __counted_by().

Consistently use "_size" suffix when the trailing '\0' is counted.
Note that MAX_RBDATA_TEXT_SIZE was originally used to limit
the text length.

When touching the code, make sure that @text_size produced by
get_random_u32_inclusive() stays within the limits.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/eaea66b9-266a-46e7-980d-33f40ad4b215@sabinyo.mountain
Suggested-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20250702095157.110916-4-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>

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printk: ringbuffer: Explain why the KUnit test ignores failed writes

The KUnit test ignores prb_reserve() failures on purpose. It tries
to push the ringbuffer beyond limits.

Note that it is a know

printk: ringbuffer: Explain why the KUnit test ignores failed writes

The KUnit test ignores prb_reserve() failures on purpose. It tries
to push the ringbuffer beyond limits.

Note that it is a know problem that writes might fail in this situation.
printk() tries to prevent this problem by:

+ allocating big enough data buffer, see log_buf_add_cpu().

+ allocating enough descriptors by using small enough average
record, see PRB_AVGBITS.

+ storing the record with disabled interrupts, see vprintk_store().

Also the amount of printk() messages is always somehow bound in
practice. And they are serialized when they are printed from
many CPUs on purpose, for example, when printing backtraces.

Reviewed-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20250702095157.110916-2-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>

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printk: ringbuffer: Add KUnit test

The KUnit test validates the correct operation of the ringbuffer.
A separate dedicated ringbuffer is used so that the global printk
ringbuffer is not touched.

Co-

printk: ringbuffer: Add KUnit test

The KUnit test validates the correct operation of the ringbuffer.
A separate dedicated ringbuffer is used so that the global printk
ringbuffer is not touched.

Co-developed-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://patch.msgid.link/20250612-printk-ringbuffer-test-v3-1-550c088ee368@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

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