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From:   John Ogness <john.ogness@linutronix.de>
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To:     Mukesh Ojha <quic_mojha@quicinc.com>, linux-kernel@vger.kernel.org
Cc:     rostedt@goodmis.org, senozhatsky@chromium.org, pmladek@suse.com,
        Mukesh Ojha <quic_mojha@quicinc.com>
Subject: Re: [PATCH] printk: Fix seq check inside devkmsg_read()
In-Reply-To: <1642505688-1194-1-git-send-email-quic_mojha@quicinc.com>
References: <1642505688-1194-1-git-send-email-quic_mojha@quicinc.com>
Date:   Tue, 18 Jan 2022 14:26:34 +0106
Message-ID: <874k61xjrx.fsf@jogness.linutronix.de>
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On 2022-01-18, Mukesh Ojha <quic_mojha@quicinc.com> wrote:
> prb_read_valid() could return a record whose seq number can be
> newer than the requested one. In that case, we allow the reading
> of the data instead of erroring out.

The EPIPE error is intentional. It informs the devkmsg reader that a
record was missed.

For simple readers, like cat(1), this means that the kernel messages
cannot be read completely reliably. We talked about this a while back
[0] because initially I wanted to do what your patch does. In the end it
was agreed that readers of /dev/kmsg must implement EPIPE handling for
missed records.

John Ogness

[0] https://lore.kernel.org/all/87pneiyv12.fsf@linutronix.de