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Tue, 27 Oct 2026 06:50:02 -0700 (PDT) Received: from pathway.suse.cz ([176.114.240.130]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e3ebcb9cesm24068407f8f.27.2026.10.27.06.50.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Oct 2026 06:50:01 -0700 (PDT) Date: Tue, 27 Oct 2026 14:50:00 +0100 From: Petr Mladek To: Wang Wensheng Cc: rostedt@goodmis.org, john.ogness@linutronix.de, senozhatsky@chromium.org, d-tatianin@yandex-team.ru, linux-kernel@vger.kernel.org Subject: Re: [PATCH] printk_ringbuffer: Fix space_used() when blk_lpos->next overflows Message-ID: References: <20261004075755.363989-1-wsw9603@163.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261004075755.363989-1-wsw9603@163.com> On Sun 2026-10-04 15:57:55, Wang Wensheng wrote: > Since commit 67e1b0052f6b ("printk_ringbuffer: don't needlessly wrap data > blocks around") blk_lpos is treated as not wrapped when blk_lpos->next > overflows to the start of data_ring. We use the indices of blk_lpos->next > and blk_lpos->start to calculate the size of the data_block. This computes > zero minus a small positive value, resulting in underflow and an > unexpectedly large value. Simply using the original lpos with wrap-counts > solves the problem. > > Fixes: 67e1b0052f6b ("printk_ringbuffer: don't needlessly wrap data blocks around") > Signed-off-by: Wang Wensheng Great catch! The fix looks good. Reviewed-by: Petr Mladek As John explained, fortunately it is not critical because the space is used only to count the free size when copying messages from the initial static ring buffer to a bigger one. And the bug will happen only when there is no free space anyway. Well, I will still queue it for 7.4 because I am going to create another pull request anyway. Best Regards, Petr