Re: reading a binary sysfs attribute continues forever

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Jonathan M. McCune wrote:
Hello,

I have written a kernel module which introduces a new subsystem in
sysfs, and it contains several attributes, one of which is binary.  So
far, I've been testing it using text.  My problem is, attempting to read
data continues forever.  For example:

# echo "test data" > /sys/mystuff/binaryattrib
# cat /sys/mystuff/binaryattrib

test data
test data
test data
test data
test data
test data
test data
test data
test data
test data
test data
test data
.. and so on forever.

Here are the read and write functions for the binary attribute:
'void *input_params' is a pointer to PAGE_SIZE (4096 for me) bytes, and
'size_t input_params_size' contains the actual number of bytes which
were written  to input_params by the _write function.  input_params_size
is initialized to zero when the module is first loaded.

static ssize_t binaryattrib_read(struct kobject *kobj, char *buf, loff_t
pos,
                           size_t count) {

    if(input_params_size + sizeof(size_t) > count)
        return -EINVAL;

    memcpy(buf, (void *)&input_params_size, sizeof(size_t));
    memcpy(buf+sizeof(size_t), input_params, input_params_size);

    return input_params_size + sizeof(size_t);

I don't know sysfs functions well but this looks wrong, you're ignoring the pos argument completely and always returning the same data. You should be copying the data starting at pos and returning only the number of bytes available from that point (possibly zero). Otherwise the user app will never get an end of file since more data is always available.

Also, you shouldn't be returning EINVAL if they try to read less than the size of your data as that is acceptable behavior and will work fine if you handle the pos argument properly.

--
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from [email protected]
Home Page: http://www.roberthancock.com/

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