Reading in kernel second time overrides first instance

I have written a read function to read a file into buffer in Kernel space.

int readfile(const char *filename, void *buf, int len, int offset) 
{
    struct file *filp;
    mm_segment_t oldfs;
    int bytes;
    filp = NULL;
    filp = filp_open(filename, O_RDONLY, 0);
    if(!filp || IS_ERR(filp)) {
        printk(" Error in reading file %s. Error = %dn", filename, 
               (int) PTR_ERR(filp));
        return -1;
    }
    filp->f_pos = offset;
    oldfs = get_fs();
    set_fs(get_ds());
    bytes = vfs_read(filp, buf, len, &filp->f_pos);
    set_fs(oldfs);
    filp_close(filp, NULL);
    return bytes;
}

Now, this function works really well and I am able to read contents of filename into buf by calling this function from my system call

char *firstbuffer;
firstbuffer = kmalloc(sizeof(PAGE_SIZE), GFP_KERNEL);

bytesread = readfile(firstfile, firstbuffer, len, 0);

// Null terminate read string
firstbuffer[bytesread] = '';
printk("first buffer = %sn",firstbuffer);

Then, I am calling this function again to read contents of secondfile into secondbuffer.

char *secondbuffer;
secondbuffer = kmalloc(sizeof(PAGE_SIZE), GFP_KERNEL);
bytesread2 = readfile(secondfile, secondbuffer, len, 0);
// Null terminate read string
secondbuffer[bytesread2] = '';
printk("second buffer %s", secondbuffer);

The problem is that after calling the read function on secondfile, the contents of my firstbuffer are getting overridden with contents of secondbuffer.

For example: if the contents of firstfile are

A
B
C

and contents of secondfile are

X
Y
Z

then after first read file call, the content of firstbuffer is:

A
B
C

and then after second read file call, the content of firstbuffer is:

A
X
Y
Z

Now, I am not sure what is going wrong here, but after second read function call, contents of firstbuffer is getting merged with contents of secondbuffer. How do I fix this?

Disclaimer:

I know we shouldn’t do file I/O in Kernel space. This is purely to learn how read functions work in Kernel space.

Answer

kmalloc(sizeof(PAGE_SIZE), GFP_KERNEL)

This allocates sizeof(PAGE_SIZE) bytes. Now, PAGE_SIZE is an integer, so it’s probably 4 bytes long, so you allocate 4 bytes.

If you wanted to allocate PAGE_SIZE bytes, use:

kmalloc(PAGE_SIZE, GFP_KERNEL)

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Answer