copy_from_user() and block_write question

[Stephane Eranian]

Hi,

Could someone explain to me what was the intent of the following
piece of code (extracted from linux-2.3.35 in fs/buffer.c) ?

int block_write_zero_range(struct inode *inode, struct page *page,
                unsigned zerofrom, unsigned from, unsigned to,
                const char * buf)
{
	....


        /*
         * Now we can copy the data.
         */
        if (zerofrom < from)
                memset(kaddr+zerofrom, 0, from-zerofrom);
        if (from < to)
>>>>            err = copy_from_user(kaddr+from, buf, to-from);
        if (to < zeroto)
                memset(kaddr+to, 0, zeroto-to);
        else
                zeroto = to;
>>>>>   if (err < 0)
                goto out;


copy_from_user() always returns something that is >= 0 so the test 
err < 0 is never taken. In this case, isn't the use of err for
copy_from_user() kind of confusing ?

I understand that POSIX does not define the behavior of write when
you have a wild pointer. That's the behavior that you get here:
write never fails because buf is invalid. I have seen systems where
the behavior is to return EFAULT though.

Any comments ?

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