[PATCH] printk: fix printk.devkmsg sysctl

[Rabin Vincent]

From: Rabin Vincent <rab...@axis.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 <rab...@axis.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