Reading large /proc entry from kernel module

From: Kristian Sørensen
Date: Tue Mar 08 2005 - 08:47:36 EST

Hi all!

I have some trouble reading a 2346 byte /proc entry from our Umbrella kernel

Proc file is created write-only and I am able to write text to the file, and
read it from kernel space. The function reading the entry is in short this:

static int umb_proc_write(struct file *file, const char *buffer,
unsigned long count, void *data) {
char *policy;
int *lbuf;
int i;

if (count != UMB_POLICY_SIZE) {
printk("Umbrella: Error - /proc/umbrella is of invalid size\n");
return -EFAULT;

/* Initialization of lbuf */
policy = kmalloc(sizeof(char)*UMB_POLICY_SIZE, GFP_ATOMIC);
lbuf = kmalloc(count, GFP_KERNEL);
if (!lbuf || !policy) {
return -EFAULT;
if (copy_from_user(lbuf, buffer, count)) {
return -EFAULT;

strcpy(policy, lbuf);


If I read byte by byte will only give the characters on every fourth index.
E.g. reading lbuf with the string "abcd", then lbuf[0]==a and lbuf[1]==d ...
- Do anyone have an explanation for this behaviour? Making the strcpy does fix
the problem - and the complete string is available! :-/ ...

Now that everything works, I want to write a string of excactly 2346
characters to the /proc/umbrella file. However when I make the
copy_from_user, I only get the first 1003 characters :-((
- Do you have a pointer to where I do this thing wrong?

What is the limit regarding the size of writing a /proc entry? (we consider
importing binary public keys to the kernel this way in the future).

Best regards,

Kristian Sørensen
- The Umbrella Project -- Security for Consumer Electronics

E-mail: ipqw@xxxxxxxxxxxx, Phone: +45 29723816
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