Messages in this thread | | | Date | Tue, 27 Nov 2018 13:44:12 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH] tracing: Fix an off by one in __next() |
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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@oracle.com> 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@oracle.com> > > 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 ;-)
Thanks!
-- Steve
> return NULL; > > m->private = (void *)n;
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