linux-kernel - Re: [PATCH v5 4/4] printk: Give error on attempt to set log buffer length to over 4G

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Message-ID: <20181009130538.7rtd3du2d7l6m63w@pathway.suse.cz>
Date:   Tue, 9 Oct 2018 15:05:38 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     zhe.he@...driver.com, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/4] printk: Give error on attempt to set log buffer
 length to over 4G

On Mon 2018-10-08 23:59:50, Sergey Senozhatsky wrote:
> On (10/08/18 15:59), Petr Mladek wrote:
> > I tried this patch with log_buf_len=5G. The kernel did not crash
> > but dmesg shown some mess. There are several 32-bit variables
> > to store the size, for example:
> > 
> > static u32 log_buf_len = __LOG_BUF_LEN;
> > u32 log_buf_len_get(void)
> > static u32 log_first_idx;
> > static u32 log_next_idx;
> > 
> > I guess that the code somewhere does not detect an overflows
> > correctly.
> > 
> > I am not motivated enought to add support for such huge message
> > buffer. Therefore I suggest to limit it to 32G for now.
> 
> Yeah, I think we gonna have problems even with a 4G logbuf and a 32-bit
> user-space doing syslog(int len).
> 
> I agree on the "not motivated enough" part ;)

OK, I have pushed an updated patch that has the limit 2GB
into printk.git, for-4.20 branch.

Note that it is slightly different than the yesterday's proposal.
I made a mistake in testing and still compared with UNIT_MAX.

The pushed version can be seen at
https://git.kernel.org/pub/scm/linux/kernel/git/pmladek/printk.git/commit/?h=for-4.20&id=e6fe3e5b7d16e8f146a4ae7fe481bc6e97acde1e

Best Regards,
Petr