linux-hardening - [bug report] printk: ringbuffer: Add KUnit test

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Message-ID: <eaea66b9-266a-46e7-980d-33f40ad4b215@sabinyo.mountain>
Date: Wed, 25 Jun 2025 10:22:19 -0500
From: Dan Carpenter <dan.carpenter@...aro.org>
To: thomas.weissschuh@...utronix.de
Cc: John Ogness <john.ogness@...utronix.de>, Kees Cook <kees@...nel.org>,
	linux-hardening@...r.kernel.org
Subject: [bug report] printk: ringbuffer: Add KUnit test

Hello Thomas Weißschuh,

The patch 5ea2bcdfbf46: "printk: ringbuffer: Add KUnit test" from Jun
12, 2025, leads to the following static checker warning:

	kernel/printk/printk_ringbuffer_kunit_test.c:91 prbtest_check_data()
	(unpublished script worries this an off by one)

kernel/printk/printk_ringbuffer_kunit_test.c
    83 static bool prbtest_check_data(const struct prbtest_rbdata *dat)
    84 {
    85 	unsigned int len;
    86 
    87 	/* Sane length? */
    88 	if (dat->len < 1 || dat->len > MAX_RBDATA_TEXT_SIZE)
    89 		return false;
    90 
--> 91 	if (dat->text[dat->len] != '\0')
    92 		return false;
    93 

My question is that the prbtest_rbdata structure is declared like this:

    53  /* test data structure */
    54  struct prbtest_rbdata {
    55          unsigned int len;
    56          char text[] __counted_by(len);
    57  };

The size of text is not really counted by len, it's "MAX_RBDATA_TEXT_SIZE
+ 1".  The condition "if (dat->text[dat->len] != '\0')" is reading one
element beyond the __counted_by() value so something should complain if
we enable all the debugging, right?

regards,
dan carpenter