How to read a kernel module (/dev) file from user space C

Question

In my kernel module the read function is as follows:

ssize_t aes_read(struct file *filp, char *buf, size_t count, loff_t *f_pos) { 

  unsigned long aes_reg[4];
  aes_reg[0] = leon_load_reg(output_mem_loc);
  aes_reg[1] = leon_load_reg(output_mem_loc+4);
  aes_reg[2] = leon_load_reg(output_mem_loc+8);
  aes_reg[3] = leon_load_reg(output_mem_loc+12);
  copy_to_user(buf, (char *)aes_reg, 16);
  ....

And it appears in the kernel module that buf is being set properly. On the user space side I have written this:

int main(int argc, char* argv[]){
    FILE *fpaes;
    char *str;
    int buf[4];

    fpaes = fopen("/dev/aes", "r");
    fread(str, 16, 1, fpaes);
    p_long = (unsigned long *)str;
    ....

But str is not being updated with the values I expect. Am I allowed to do an fread in this way or am I way off?

This isn't causing your problem, but not that it's generally preferable to directly call the system calls read(2) et al instead of going through the stdio layer, since stdio buffers reads and writes by default. — Adam Rosenfield
– Adam Rosenfield, Commented Aug 23, 2012 at 18:50

ouah · Accepted Answer · 2012-08-23 18:49:02Z

3

str is not initialized in your program. Accessing *str is undefined behavior.

answered Aug 23, 2012 at 18:49

ouah

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1 Comment

Stuart Over a year ago

Yep, I must have missed that. Thanks.

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