String length limit for a kernel module parameter

Question

I am passing a string as parameter when loading a kernel module. When the string is > 1024 chars, modprobe results in an error:

FATAL: Error inserting mymodule (/lib/modules..): No space left on device

dmesg output:

mystr: string parameter too long

Are module parameters limited to 1024 char strings?

A better question might be "Is there a better solution than passing a 1024 char string as a parameter", or then again maybe not. Could you explain why this is necessary? There may well be better approaches. — terdon
– terdon, Commented Mar 20, 2014 at 14:01
The string represents a binary code, which I use for computations in my module. A string seems to be the best solution regarding input convenience and later computations. — goose999
– goose999, Commented Mar 20, 2014 at 14:28
Why don't you split it into two, pass each part as a separate parameter and join it in your code? — terdon
– terdon, Commented Mar 20, 2014 at 14:29
Will do now. I also came up with this, but I would have preferred having only 1 parameter for the code. — goose999
– goose999, Commented Mar 20, 2014 at 14:38
You might want to obtain the argument in some other way, e.g. through /sys after loading the module. — Gilles 'SO- stop being evil'
– Gilles 'SO- stop being evil', Commented Mar 20, 2014 at 23:56

cuonglm · Accepted Answer · 2014-03-20 14:11:28Z

4

I think not only module but also all linux command kernel parameters are limited to 1024 char. From linux source code, file kernel/params.c:

int param_set_charp(const char *val, const struct kernel_param *kp)
{
    if (strlen(val) > 1024) {
        pr_err("%s: string parameter too long\n", kp->name);
        return -ENOSPC;
    }

    maybe_kfree_parameter(*(char **)kp->arg);

    /* This is a hack.  We can't kmalloc in early boot, and we
     * don't need to; this mangled commandline is preserved. */
    if (slab_is_available()) {
        *(char **)kp->arg = kmalloc_parameter(strlen(val)+1);
        if (!*(char **)kp->arg)
            return -ENOMEM;
        strcpy(*(char **)kp->arg, val);
    } else
        *(const char **)kp->arg = val;

    return 0;
}

So the answer, you can not pass parameter which bigger than 1024 chars.

answered Mar 20, 2014 at 14:11

cuonglm

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2 Comments

Graeme Over a year ago

You can definitely have more than 1024 bytes in a noormal command argument. Consider the following - /bin/echo $(tr -dc [:alnum:] </dev/urandom | head -c 1025) - here we have a 1025 byte argument

cuonglm Over a year ago

Yeap, my mistypo. I means kernel paramaters.

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