How to enable WARN_ON when building kernels with make

[Bruno Wolff III]

I need to get some WARN_ON output to help a developer figure out a 
regression in 4.15. When I test the problem with kernels from Fedora 
built rpms I get the output from WARN_ON, but they don't have a 
debugging patch from the developer applied. When I build using make 
(both Linus' tree and Boyer's tree) I don't get that output.
I'm guessing there is a config setting that isn't in the normal Fedora 
config file or I need to force a gcc option or something. But I thought 
someone here might just know what a need to know. Otherwise I'll try to 
figure it out from the spec file, but my memory of the kernel spec file 
is that it is complicated and it might not be easy to figure out what 
I'm looking for.
Thanks.

[Bruno Wolff III]

It looks like .config has reasonable (the same as Fedora) values for 
CONFIG_BUG, CONFIG_CONSOLE_LOGLEVEL_DEFAULT and 
CONFIG_MESSAGE_LOGLEVEL_DEFAULT.

The loglevel kernel parameter is not used, so the defaults should apply.

When I get physical access to the test machine on Monday I can try setting 
loglevel on boot, which might work in the WARN_ON code was actually 
compiled in. My guess is that it isn't, but I don't know why.

[stan]

On Sat, 16 Dec 2017 10:18:27 -0600
Bruno Wolff III <bruno(a)wolff.to&gt; wrote:

>  It looks like .config has reasonable (the same as Fedora) values for
>
>  CONFIG_BUG, CONFIG_CONSOLE_LOGLEVEL_DEFAULT and 
>  CONFIG_MESSAGE_LOGLEVEL_DEFAULT.
>  
>  The loglevel kernel parameter is not used, so the defaults should
>  apply.
>  
>  When I get physical access to the test machine on Monday I can try
>  setting loglevel on boot, which might work in the WARN_ON code was
>  actually compiled in. My guess is that it isn't, but I don't know why. 
It seems to be that WARN_ON is a macro, so is always available.

tools/include/asm/bug.h:#define WARN_ON(condition) ({                                   \
tools/include/asm/bug.h:        int __ret_warn_on = !!(condition);                      \
tools/include/asm/bug.h:        if (unlikely(__ret_warn_on))                            \
tools/include/asm/bug.h:        unlikely(__ret_warn_on);                                \

Is it possible you just need to tell the system to display the messages
from the kernel?

sysctrl kernel.printk

should show you the current levels.  From memory, higher numbers mean
less priority for what is displayed.  I set to 3 4 1 3.  

sysctrl -w kernel.printk "3 4 1 3"

You could try upping these to 7 (the max) to see what happens.  Or, if
you are inclined, you could just read the man page to see if this
has anything to do with what you want.  :-)