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

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Date:   Mon, 18 Sep 2017 19:08:49 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        pierre kuo <vichy.kuo@...il.com>,
        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 (09/18/17 03:00), Joe Perches wrote:
[..]
> > 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

linked modules are not printed as a single line - printk("module linked in ................................").


it's a bunch of pr_cont() calls:

	printk(KERN_DEFAULT "Modules linked in:");
	list_for_each_entry_rcu(mod, &modules, list) {
		pr_cont(" %s%s", mod->name, module_flags(mod, buf))
	}


when pr_cont() buffer is getting full, we just flush it (log_store())

	if (cont.len + len > sizeof(cont.buf))
		cont_flush();


but the point is -- we don't truncate anything
here, and don't lose the content. don't we?

	-ss