Re: [PATCH 07/17] usb: host: ehci-dbg: fix unsigned comparison

[Alan Stern]

On Mon, 4 Jan 2016, Geyslan G. Bem wrote:

> 2016-01-04 17:50 GMT-03:00 Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>:
> > On Mon, 4 Jan 2016, Geyslan G. Bem wrote:
> >
> >> This patch fixes an unsigned comparison to less than 0.
> >
> > No, it doesn't.  It changes an unsigned comparison for less than or
> > equal to 0, which is very different from less than 0.
> You're right. The statemant is incomplete.
> 
> >
> >> Signed-off-by: Geyslan G. Bem <geyslan@xxxxxxxxx>
> >> ---
> >>
> >> Notes:
> >>     I'm not sure about that comparison because in qh_lines() temp receives
> >>     the snprintf() return and thereafter occurs this comparison:
> >>
> >>     if (size < temp)
> >>                  temp = size;
> >>
> >>     Is it a test of string truncation right? That possibility is being
> >>     treated. But if after some snprintf returns the temp is exactly size
> >>     minus 1 (trailing null)? Could this scenario happen? If yes, I think
> >>     size could be not counting correctly. Let me know more about it.
> >
> > I think the two weird code sequences in qh_lines() were written before
> > scnprintf existed.  They should be changed to use scnprintf instead of
> > snprintf; then the "if (size < temp) temp = size;" things can be
> > removed.
> I see. I can do another patch for that if you allow me.

Sure, go ahead.

> >>  drivers/usb/host/ehci-dbg.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/usb/host/ehci-dbg.c b/drivers/usb/host/ehci-dbg.c
> >> index 980ca55..1645120 100644
> >> --- a/drivers/usb/host/ehci-dbg.c
> >> +++ b/drivers/usb/host/ehci-dbg.c
> >> @@ -542,7 +542,7 @@ static ssize_t fill_async_buffer(struct debug_buffer *buf)
> >>               next += temp;
> >>
> >>               list_for_each_entry(qh, &ehci->async_unlink, unlink_node) {
> >> -                     if (size <= 0)
> >> +                     if (size == 0)
> >>                               break;
> >>                       qh_lines(ehci, qh, &next, &size);
> >>               }
> >
> > The new line does exactly the same thing as the old line.  There's no
> > reason to make this change.
> I think that the original and new logic will be the same because the
> size variable has no sign. If in some previous subtraction the
> subtracted value is greater than size value, this will spin (rotate),
> probably, to a great positive value.
> 
> The compiled code will not change indeed. That change was only focused
> on the improvement of the code reading. So if you allow me I could
> change the commit message. If not let's forget it. :-)

IMO there's no improvement in reading the code.  So let's forget about 
this.

Alan Stern

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