From: Steven Rostedt <rostedt@goodmis.org>
To: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org
Subject: Re: [PATCH] tracing: Fix an off by one in __next()
Date: Tue, 27 Nov 2018 13:44:12 -0500 [thread overview]
Message-ID: <20181127134412.6f2141d4@gandalf.local.home> (raw)
In-Reply-To: <20180620110758.crunhd5bfep7zuiz@kili.mountain>
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;
next prev parent reply other threads:[~2018-11-27 18:44 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-20 11:08 [PATCH] tracing: Fix an off by one in __next() Dan Carpenter
2018-11-27 18:44 ` Steven Rostedt [this message]
2018-11-27 20:04 ` Dan Carpenter
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