linux-kernel - Re: [PATCH v2] printk: Do not lose last line in kmsg buffer dump

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Date:   Fri, 12 Jul 2019 11:12:51 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     sergey.senozhatsky@...il.com, rostedt@...dmis.org,
        Vincent Whitchurch <rabinv@...s.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] printk: Do not lose last line in kmsg buffer dump

On Thu 2019-07-11 16:29:37, Vincent Whitchurch wrote:
> kmsg_dump_get_buffer() is supposed to select all the youngest log
> messages which fit into the provided buffer.  It determines the correct
> start index by using msg_print_text() with a NULL buffer to calculate
> the size of each entry.  However, when performing the actual writes,
> msg_print_text() only writes the entry to the buffer if the written len
> is lesser than the size of the buffer.  So if the lengths of the
> selected youngest log messages happen to precisely fill up the provided
> buffer, the last log message is not included.
> 
> We don't want to modify msg_print_text() to fill up the buffer and start
> returning a length which is equal to the size of the buffer, since
> callers of its other users, such as kmsg_dump_get_line(), depend upon
> the current behaviour.
> 
> Instead, fix kmsg_dump_get_buffer() to compensate for this.
> 
> For example, with the following two final prints:
> 
> [    6.427502] AAAAAAAAAAAAA
> [    6.427769] BBBBBBBB12345
> 
> A dump of a 64-byte buffer filled by kmsg_dump_get_buffer(), before this
> patch:
> 
>  00000000: 3c 30 3e 5b 20 20 20 20 36 2e 35 32 32 31 39 37  <0>[    6.522197
>  00000010: 5d 20 41 41 41 41 41 41 41 41 41 41 41 41 41 0a  ] AAAAAAAAAAAAA.
>  00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>  00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 
> After this patch:
> 
>  00000000: 3c 30 3e 5b 20 20 20 20 36 2e 34 35 36 36 37 38  <0>[    6.456678
>  00000010: 5d 20 42 42 42 42 42 42 42 42 31 32 33 34 35 0a  ] BBBBBBBB12345.
>  00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>  00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
> 
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>

I think that I need vacation. I have got lost in all the checks
and got it wrongly in the morning.

This patch fixes the calculation of messages that might fit
into the buffer. It makes sure that the function that writes
the messages will really allow to write them.

It seems to be the correct fix.

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr