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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	stable@vger.kernel.org,
	Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Guenter Roeck <linux@roeck-us.net>,
	Petr Mladek <pmladek@suse.com>, Yi Fan <yfa@google.com>
Subject: [PATCH 4.14 13/22] printk/console: Allow to disable console output by using console="" or console=null
Date: Wed, 10 Nov 2021 19:43:33 +0100	[thread overview]
Message-ID: <20211110182003.095006247@linuxfoundation.org> (raw)
In-Reply-To: <20211110182002.666244094@linuxfoundation.org>

From: Petr Mladek <pmladek@suse.com>

commit 3cffa06aeef7ece30f6b5ac0ea51f264e8fea4d0 upstream.

The commit 48021f98130880dd74 ("printk: handle blank console arguments
passed in.") prevented crash caused by empty console= parameter value.

Unfortunately, this value is widely used on Chromebooks to disable
the console output. The above commit caused performance regression
because the messages were pushed on slow console even though nobody
was watching it.

Use ttynull driver explicitly for console="" and console=null
parameters. It has been created for exactly this purpose.

It causes that preferred_console is set. As a result, ttySX and ttyX
are not used as a fallback. And only ttynull console gets registered by
default.

It still allows to register other consoles either by additional console=
parameters or SPCR. It prevents regression because it worked this way even
before. Also it is a sane semantic. Preventing output on all consoles
should be done another way, for example, by introducing mute_console
parameter.

Link: https://lore.kernel.org/r/20201006025935.GA597@jagdpanzerIV.localdomain
Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20201111135450.11214-3-pmladek@suse.com
Cc: Yi Fan <yfa@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 kernel/printk/printk.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2092,8 +2092,15 @@ static int __init console_setup(char *st
 	char *s, *options, *brl_options = NULL;
 	int idx;
 
-	if (str[0] == 0)
+	/*
+	 * console="" or console=null have been suggested as a way to
+	 * disable console output. Use ttynull that has been created
+	 * for exacly this purpose.
+	 */
+	if (str[0] == 0 || strcmp(str, "null") == 0) {
+		__add_preferred_console("ttynull", 0, NULL, NULL);
 		return 1;
+	}
 
 	if (_braille_console_setup(&str, &brl_options))
 		return 1;



  parent reply	other threads:[~2021-11-10 18:48 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-11-10 18:43 [PATCH 4.14 00/22] 4.14.255-rc1 review Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 01/22] scsi: core: Put LLD module refcnt after SCSI device is released Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 02/22] media: firewire: firedtv-avc: fix a buffer overflow in avc_ca_pmt() Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 03/22] mm/zsmalloc: Prepare to variable MAX_PHYSMEM_BITS Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 04/22] arch: pgtable: define MAX_POSSIBLE_PHYSMEM_BITS where needed Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 05/22] ARM: 9120/1: Revert "amba: make use of -1 IRQs warn" Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 06/22] IB/qib: Use struct_size() helper Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 07/22] IB/qib: Protect from buffer overflow in struct qib_user_sdma_pkt fields Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 08/22] block: introduce multi-page bvec helpers Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 09/22] Revert "x86/kvm: fix vcpu-id indexed array sizes" Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 10/22] usb: gadget: Mark USB_FSL_QE broken on 64-bit Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 11/22] usb: musb: Balance list entry in musb_gadget_queue Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 12/22] usb-storage: Add compatibility quirk flags for iODD 2531/2541 Greg Kroah-Hartman
2021-11-10 18:43 ` Greg Kroah-Hartman [this message]
2021-11-10 18:43 ` [PATCH 4.14 14/22] isofs: Fix out of bound access for corrupted isofs image Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 15/22] comedi: dt9812: fix DMA buffers on stack Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 16/22] comedi: ni_usb6501: fix NULL-deref in command paths Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 17/22] comedi: vmk80xx: fix transfer-buffer overflows Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 18/22] comedi: vmk80xx: fix bulk-buffer overflow Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 19/22] comedi: vmk80xx: fix bulk and interrupt message timeouts Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 20/22] staging: r8712u: fix control-message timeout Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 21/22] staging: rtl8192u: fix control-message timeouts Greg Kroah-Hartman
2021-11-10 18:43 ` [PATCH 4.14 22/22] rsi: fix control-message timeout Greg Kroah-Hartman
2021-11-11 18:26 ` [PATCH 4.14 00/22] 4.14.255-rc1 review Naresh Kamboju
2021-11-12  0:58 ` Guenter Roeck
2021-11-12 15:41 ` Jon Hunter

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