procfs uglyness caused by "cat"

[Herbert Rosmanith <kernel@wildsau.enemy.org>]

hello list,

when doing "cat /proc/uptime" or similar files, the
routine which prepares the buffer gets called twice.

this is because "cat" reads two times from the file,
as a system-call-trace shows:

    # strace -f cat /proc/uptime
    ...
    read(3, "5241.09 5082.74\n", 1024)      = 16
    ...
    read(3, "", 1024)                       = 0
    ...

this leads to uptime_read_proc() being called two times,
the second time with parameter off=16, but since this
is ignored, the work is done twice: clock_...gettime(),
cputime_to_timespec(), sprintf(), proc_calc_metrics()
and so on.

insert a printk-statement if you don't beleive this.

btw, there's no wrong information produced because of this,
because *page points to the same location both calls.

a simple way to get rid of this:

static int uptime_read_proc(char *page, char **start, off_t off,
                                 int count, int *eof, void *data)
{
        struct timespec uptime;
        struct timespec idle;
        int len;
        cputime_t idletime;

+	if (off)
+		return 0;

        cputime_add(init_task.utime, init_task.stime);
        do_posix_clock_monotonic_gettime(&uptime);
        cputime_to_timespec(idletime, &idle);
        len = sprintf(page,"%lu.%02lu %lu.%02lu\n",
...
and so on.

this affects possibly all /proc files which ignore the offset parameter
and are evaluated(?) with "cat".

regards,
h.rosmanith