Re: [PATCH] printk: handle blank console arguments passed in.

[Guenter Roeck <linux@roeck-us.net>]

On 10/5/20 7:59 PM, Sergey Senozhatsky wrote:
> Cc-ing Guenter,
> 
> On (20/05/22 12:00), Petr Mladek wrote:
>> On Fri 2020-05-22 16:53:06, Shreyas Joshi wrote:
>>> If uboot passes a blank string to console_setup then it results in a trashed memory.
>>> Ultimately, the kernel crashes during freeing up the memory. This fix checks if there
>>> is a blank parameter being passed to console_setup from uboot.
>>> In case it detects that the console parameter is blank then
>>> it doesn't setup the serial device and it gracefully exits.
>>>
>>> Signed-off-by: Shreyas Joshi <shreyas.joshi@biamp.com>
>>> ---
>>>  V1:
>>>     Fixed console_loglevel to default as per the review comments
>>>
>>>  kernel/printk/printk.c | 5 ++++-
>>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>>> index ad4606234545..e9ad730991e0 100644
>>> --- a/kernel/printk/printk.c
>>> +++ b/kernel/printk/printk.c
>>> @@ -2165,7 +2165,10 @@ static int __init console_setup(char *str)
>>>  	char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for "ttyS" */
>>>  	char *s, *options, *brl_options = NULL;
>>>  	int idx;
>>> -
>>> +	if (str[0] == 0) {
>>> +		return 1;
>>> +	}
>>>  	if (_braille_console_setup(&str, &brl_options))
>>>  		return 1;
>>
>> I have fixed formatting and pushed it into printk/linux.git,
>> branch for-5.8.
> 
> Petr, this patch's causing regressions for us. We use blank console= boot
> param to bypass dts. It appears that it'd be better to revert the change.
> 

Not just to bypass dts, it was also possible to use console= to disable consoles
passed as config option, as well as other default console options. A quick test
confirms that this affects all platforms/architectures, not just Chromebooks.
Prior to this patch, it was possible to disable a default console with an
empty "console=" parameter. This is no longer possible. This means that
this patch results in a substantial (and, as far as I can see, completely
undiscussed) functionality change.

I don't understand why (yet), but the patch also causes regressions with
seemingly unrelated functionality, specifically with dm-verity on at least
one Chromebook platform. I filed crbug.com/1135157 to track the problem,
and reverted the patch from all our stable releases immediately after
the last round of stable release merges.

On a side note, I don't see the problem presumably fixed with this
patch in any of my tests.

Guenter