linux-kernel - [PATCH] printk: fix printk.devkmsg sysctl

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Date:   Fri, 27 Jan 2017 14:11:46 +0100
From:   Rabin Vincent <rabin.vincent@...s.com>
To:     akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org, bp@...e.de,
        Rabin Vincent <rabinv@...s.com>
Subject: [PATCH] printk: fix printk.devkmsg sysctl

From: Rabin Vincent <rabinv@...s.com>

The comment says that it doesn't want to accept trailing crap but that's
just what it allows:

 # echo -n offX > /proc/sys/kernel/printk_devkmsg
 #

while at the same time it rejects legitimate uses:

 # echo -n off > /proc/sys/kernel/printk_devkmsg
 -sh: echo: write error: Invalid argument

Fix it.

Before this patch:

 # cat /proc/sys/kernel/printk_devkmsg
 ratelimit
 # echo off > /proc/sys/kernel/printk_devkmsg
 # sysctl -w kernel.printk_devkmsg=off
 sysctl: short write
 # echo -n off > /proc/sys/kernel/printk_devkmsg
 -sh: echo: write error: Invalid argument
 # echo -n offX > /proc/sys/kernel/printk_devkmsg
 #
 # printf "off\nX" >/proc/sys/kernel/printk_devkmsg
 -sh: printf: write error: Invalid argument

After this patch:

 # cat /proc/sys/kernel/printk_devkmsg
 ratelimit
 # echo off > /proc/sys/kernel/printk_devkmsg
 # sysctl -w kernel.printk_devkmsg=off
 kernel.printk_devkmsg = off
 # echo -n off > /proc/sys/kernel/printk_devkmsg
 # echo -n offX > /proc/sys/kernel/printk_devkmsg
 -sh: echo: write error: Invalid argument
 # printf "off\nX" >/proc/sys/kernel/printk_devkmsg
 #

As a side effect, the "off\nX" case is not rejected anymore, but that is how
the other sysctl files behave:

 # cat /proc/sys/kernel/printk_ratelimit
 0
 # printf "5\nX" >/proc/sys/kernel/printk_ratelimit
 # cat /proc/sys/kernel/printk_ratelimit
 5

Fixes: 750afe7babd117d ("printk: add kernel parameter to control writes to /dev/kmsg")
Signed-off-by: Rabin Vincent <rabinv@...s.com>
---
 kernel/printk/printk.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 8b26964..032b9e0 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -107,15 +107,15 @@ static int __control_devkmsg(char *str)
 	if (!str)
 		return -EINVAL;
 
-	if (!strncmp(str, "on", 2)) {
+	if (!strcmp(str, "on")) {
 		devkmsg_log = DEVKMSG_LOG_MASK_ON;
-		return 2;
-	} else if (!strncmp(str, "off", 3)) {
+		return 0;
+	} else if (!strcmp(str, "off")) {
 		devkmsg_log = DEVKMSG_LOG_MASK_OFF;
-		return 3;
-	} else if (!strncmp(str, "ratelimit", 9)) {
+		return 0;
+	} else if (!strcmp(str, "ratelimit")) {
 		devkmsg_log = DEVKMSG_LOG_MASK_DEFAULT;
-		return 9;
+		return 0;
 	}
 	return -EINVAL;
 }
@@ -177,7 +177,7 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
 		 * Do not accept an unknown string OR a known string with
 		 * trailing crap...
 		 */
-		if (err < 0 || (err + 1 != *lenp)) {
+		if (err < 0) {
 
 			/* ... and restore old setting. */
 			devkmsg_log = old;
-- 
2.1.4