On Tue, Nov 27, 2018 at 01:44:12PM -0500, Steven Rostedt wrote:
>
> Doing the sweep of my INBOX, I came across this patch (it was sent
> while I was in the Alps :-)
>
>
> On Wed, 20 Jun 2018 14:08:00 +0300
> Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
>
> > The > should be >= to prevent an off by one bug.
>
> Well, not really.
>
> >
> > >From reviewing the code, it seems possible for
> > stack_trace_max.nr_entries to be set to .max_entries and in that case we
> > would be reading one element beyond the end of the stack_dump_trace[]
> > array. If it's not set to .max_entries then the bug doesn't affect
> > runtime.
> >
> > Signed-off-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> >
> > diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
> > index 4237eba4ef20..6e3edd745c68 100644
> > --- a/kernel/trace/trace_stack.c
> > +++ b/kernel/trace/trace_stack.c
> > @@ -286,7 +286,7 @@ __next(struct seq_file *m, loff_t *pos)
> > {
> > long n = *pos - 1;
> >
> > - if (n > stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX)
> > + if (n >= stack_trace_max.nr_entries || stack_dump_trace[n] == ULONG_MAX)
>
> We have:
>
> static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES+1] =
> { [0 ... (STACK_TRACE_ENTRIES)] = ULONG_MAX };
>
> And
>
> struct stack_trace stack_trace_max = {
> .max_entries = STACK_TRACE_ENTRIES - 1,
> .entries = &stack_dump_trace[0],
> };
>
>
> And nr_entries is set as this, and we have after that this:
>
> stack_trace_max.nr_entries = x;
> for (; x < i; x++)
> stack_dump_trace[x] = ULONG_MAX;
>
> Where we set stack_dump_trace[nr_entries] to ULONG_MAX.
>
> Thus, nr_entries will not go pass the size of stack_dump_trace.
>
> That said, if n == nr_entries, the second part of that if will always
> be true. And this is a bit subtle, so I will apply the patch. But it is
> not an off by one bug ;-)
Ah, yes. I follow that now. Thanks for taking the time to review this
patch.
I am optimistic that eventually I will fix how Smatch handles loops so
it maybe will be able to figure out that "x <= STACK_TRACE_ENTRIES - 1"
but that's probably some time off.
regards,
dan carpenter
