[PATCH 5.17 0380/1126] printk: fix return value of printk.devkmsg __setup handler

From: Greg Kroah-Hartman
Date: Tue Apr 05 2022 - 04:36:08 EST

From: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>

[ Upstream commit b665eae7a788c5e2bc10f9ac3c0137aa0ad1fc97 ]

If an invalid option value is used with "printk.devkmsg=<value>",
it is silently ignored.
If a valid option value is used, it is honored but the wrong return
value (0) is used, indicating that the command line option had an
error and was not handled. This string is not added to init's
environment strings due to init/main.c::unknown_bootoption()
checking for a '.' in the boot option string and then considering
that string to be an "Unused module parameter".

Print a warning message if a bad option string is used.
Always return 1 from the __setup handler to indicate that the command
line option has been handled.

Fixes: 750afe7babd1 ("printk: add kernel parameter to control writes to /dev/kmsg")
Signed-off-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Reported-by: Igor Zhbanov <i.zhbanov@xxxxxxxxxxxx>
Link: lore.kernel.org/r/64644a2f-4a20-bab3-1e15-3b2cdd0defe3@xxxxxxxxxxxx
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: John Ogness <john.ogness@xxxxxxxxxxxxx>
Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Reviewed-by: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Link: https://lore.kernel.org/r/20220228220556.23484-1-rdunlap@xxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
kernel/printk/printk.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 82abfaf3c2aa..833e407545b8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -146,8 +146,10 @@ static int __control_devkmsg(char *str)

static int __init control_devkmsg(char *str)
- if (__control_devkmsg(str) < 0)
+ if (__control_devkmsg(str) < 0) {
+ pr_warn("printk.devkmsg: bad option string '%s'\n", str);
return 1;
+ }

* Set sysctl string accordingly:
@@ -166,7 +168,7 @@ static int __init control_devkmsg(char *str)
devkmsg_log |= DEVKMSG_LOG_MASK_LOCK;

- return 0;
+ return 1;
__setup("printk.devkmsg=", control_devkmsg);