linux-kernel - Re: [RFC V2] printk: add warning while drop partial text in msg

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Date:   Mon, 18 Sep 2017 03:00:03 -0700
From:   Joe Perches <joe@...ches.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        pierre kuo <vichy.kuo@...il.com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        rostedt@...dmis.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V2] printk: add warning while drop partial text in msg

On Mon, 2017-09-18 at 18:39 +0900, Sergey Senozhatsky wrote:
> Hi,
> 
> On (09/12/17 14:07), pierre kuo wrote:
> [..]
> > int per_cpu_thread_fn(void* data)
> > {
> >     unsigned int index = 0;
> >     unsigned int len = 0;
> >     char* local_string = kzalloc(2048, GFP_KERNEL);
> > 
> >     do {
> >         len += sprintf((local_string + len), "this is the %d line\n", index++);
> >     }while(len < 576);
> >     printk_deferred("%s", local_string);
> >     return 0;
> > }
> 
> which is not a real world example, isn't it?
> 
> printk_deferred("%s", local_string[2048]) makes no sense anyway,
> since we limit the message size to 1024 - HEADER chars in
> vprintk_emit()  // see static char textbuf[LOG_LINE_MAX].
> 
> I'm not quite following what were you trying to prove, sorry.
> does any function in the upstream kernel printk()-s buffers
> larger than LOG_LINE_MAX? which one?
> 
> 
> - the longest line in my dmesg output is 260 bytes in total (including timestamp)

try a kernel with a few modules loaded

For instance:

$ dmesg | awk '{ print length($0), $0 }'|sort -rn | head -1
827 [337318.968616] Modules linked in: snd_usb_audio snd_usbmidi_lib ccm rfcomm bnep snd_hda_codec_hdmi dell_led snd_hda_codec_realtek snd_hda_codec_generic hid_multitouch binfmt_misc nls_iso8859_1 arc4 i2c_designware_platform i2c_designware_core dell_wmi snd_soc_skl snd_soc_skl_ipc snd_soc_sst_ipc snd_soc_sst_dsp snd_hda_ext_core snd_soc_sst_match snd_soc_core snd_compress ac97_bus snd_pcm_dmaengine snd_hda_intel dell_laptop intel_rapl snd_hda_codec dell_smbios dcdbas x86_pkg_temp_thermal snd_hda_core intel_powerclamp snd_hwdep coretemp kvm_intel snd_pcm kvm irqbypass crct10dif_pclmul crc32_pclmul ghash_clmulni_intel iwlmvm pcbc mac80211 snd_seq_midi snd_seq_midi_event aesni_intel aes_x86_64 crypto_simd snd_rawmidi glue_helper cryptd intel_cstate intel_rapl_perf snd_seq iwlwifi snd_seq_device uvcvideo videobuf2_vmalloc