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Passing a custom parameter to the kernel

Hi,

I'm hacking some code in the kernel, and want to pass in a custom
parameter to the kernel (either at boot time, or after startup),
which my new code will use. This parameter is a number, ranging from
40 to 800, for example.

The existing boot command line parameters do not allow the use of a
user defined custom parameter to be passed in to the kernel. Is there
a way to do this, either during or after boot?

Thanks,
John

Have you tried using module_param()?

As far I know, any string you pass to kernel at boot time is available in /proc/cmdline pseudo file.
Check your current /proc/cmdline and you will find all strings you kernel received at boot time, even those are user space specific or are not used by anyone.

Quote:

Originally Posted by marozsas (Post 2905733)

Thanks for the tips, Osor and Marozsas. I haven't had time to try out
either one. I will post what I find after looking into it some more.

John

Quote:

Originally Posted by osor (Post 2905693)

Have you tried using module_param()?

Hi Osor,

I was looking into module_param, but it appears that it is used mainly
for modules. There seems to be no documentation on what needs to be
done if I wanted to add a parameter to the "kernel" boot command line
in grub.conf, that would be visible as a global after init is done.

John

Quote:

Originally Posted by jkoshi (Post 2917349)

There seems to be no documentation on what needs to be
done if I wanted to add a parameter to the "kernel" boot command line
in grub.conf, that would be visible as a global after init is done.

The module_param() macro is indeed for modules. If your “module” is not built as a module, but actually built-in to the kernel, you may still use the macro. In this case, you pass the parameter through the kernel command line using the modulename, followed by a dot (“.”), followed by the parameter. This is all documented here.

For example, consider the loopback block device module (loop.ko). This module takes a single optional parameter (max_loop) telling it the maximum number of loopback devices. So if I want to alter this parameter at the time of module loading, I would do something like this:

Code:

# modprobe loop max_loop=32

Suppose, however, that I have compiled the loopback block device “module” into my kernel. Then, I might pass the following line to the kernel at boot:

Code:

kernel /vmlinuz root=/dev/sda3 loop.max_loop=32

You can also assign permissions such that the file “/sys/module/loop/parameters/max_loop” is readable, is writable, is both, or is neither. In this case, the permissions are 0 (so the sysfs-interface file simply doesn’t exist).

If you want to be slightly more arcane about kernel parameters for built-in code, you can always use the __setup() macro. If you want to be slightly more sophisticated, you could create a sysctl interface.

Quote:

Originally Posted by jkoshi (Post 2907012)

Thanks for the tips, Osor and Marozsas. I haven't had time to try out
either one. I will post what I find after looking into it some more.

You don't tell us if you have tried it already, but if you start your linux with

Code:

kernel /vmlinuz ro root=LABEL=/ rhgb quiet loop.max_loop=32 myparam=45

then, in the file /proc/cmdline you will have:

Code:

[root@agente86 ~]# cat /proc/cmdline 

ro root=LABEL=/ rhgb quiet loop.max_loop=32 myparam=45

[root@agente86 ~]#

and you can inspect and parse the file contents in usual ways, e.g.:

Code:

[root@agente86 ~]# for arg in $(cat /proc/cmdline); do  

    echo $arg | grep -q myparam ; 

    if [ $? == 0 ]; then      

        val=$(echo $arg | cut -d= -f2);    

        echo "myparm is $val"; 

    fi; 

done

myparm is 45

[root@agente86 ~]#

Using this technique you can pass any string at boot time to any program in user space, like init scripts, /etc/rc.whatever scripts....

Quote:

Originally Posted by osor (Post 2917455)

Thanks Osor,

Since this is a hack for a temporary test system, I decided to use the
__setup macro and get_option() in init/main.c, as follows:

Code:

unsigned int my_param;

EXPORT_SYMBOL (my_param);

.

.

.

static int __init set_my_param(char *str)

{

    get_option(&str, &my_param);

    return 1;

}



__setup("my_param=", set_my_param);

I then add "my_param=<value>" to the "kernel" line in grub.conf,
and in the kernel code areas where I need to use this parameter,
I extern my_param and use it;

I'm still not clear how I would use module_param() in the case where
it is not a parameter for a single module, but rather a value that I
need to see and use in several kernel code areas, ie: mm and fs, for
example.

Quote:

Originally Posted by marozsas (Post 2917469)

You don't tell us if you have tried it already, but if you start your linux with...

Thanks marozsas,

I wanted to add that in the current kernel, the command line
is also available as a global:

Code:

char *saved_command_line

Since strtok is available as a kernel function, this string can be
easily parsed in C code in the kernel, to get any user parameter that
is added to the "kernel" line of the grub configuration file. Please
see my earlier post for what I eventually used.

Sorry !

I didn't realize you are talking about coding in kernel space.
I had to read your post a second time to figure out that, and it is very clear at beginning of your original post - my bad ....

have a happy hacking,

Quote:

Originally Posted by jkoshi (Post 2924990)

Thanks Osor,

Since this is a hack for a temporary test system, I decided to use the
__setup macro and get_option() in init/main.c, as follows:

Code:

unsigned int my_param;

EXPORT_SYMBOL (my_param);

.

.

.

static int __init set_my_param(char *str)

{

    get_option(&str, &my_param);

    return 1;

}



__setup("my_param=", set_my_param);

I then add "my_param=<value>" to the "kernel" line in grub.conf,
and in the kernel code areas where I need to use this parameter,
I extern my_param and use it;

how do i extern my_param and use it?
and is a specific point i put this function in main.c or not?

Quote:

Originally Posted by othe (Post 3603632)

how do i extern my_param and use it?
and is a specific point i put this function in main.c or not?

Hi,

Declare the function in main.c like the other __init functions like nosmp, maxcpus, etc, and then the related __setup() function below that, for clarity. In the code area where you want to use this parameter, declare "extern my_param" up top, so you can use it.

Hope this helps.

John