linux-kernel - Re: [PATCH] ftrace: unsigned idx cannot be less than 0

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Date:	Tue, 6 Jan 2009 16:58:29 +0100
From:	"Frédéric Weisbecker" <fweisbec@...il.com>
To:	"Steven Rostedt" <rostedt@...dmis.org>
Cc:	"Roel Kluin" <roel.kluin@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	"Ingo Molnar" <mingo@...e.hu>,
	"Liming Wang" <liming.wang@...driver.com>
Subject: Re: [PATCH] ftrace: unsigned idx cannot be less than 0

2009/1/6 Steven Rostedt <rostedt@...dmis.org>:
>
> [ added Liming to CC ]
>
> On Fri, 2 Jan 2009, Frederic Weisbecker wrote:
>
>> On Fri, Jan 02, 2009 at 03:49:43PM +0100, Roel Kluin wrote:
>> > // vi kernel/trace/ftrace.c +787
>> > struct ftrace_iterator {
>> > ...
>> >         unsigned                idx;
>> > ...
>> > };
>> >
>> > idx is unsigned and cannot be less than 0.
>> >
>> > Signed-off-by: Roel Kluin <roel.kluin@...il.com>
>> > ---
>> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
>> > index 2f32969..a344add 100644
>> > --- a/kernel/trace/ftrace.c
>> > +++ b/kernel/trace/ftrace.c
>> > @@ -842,7 +842,7 @@ static void *t_start(struct seq_file *m, loff_t *pos)
>> >     void *p = NULL;
>> >
>> >     if (*pos > 0) {
>> > -           if (iter->idx < 0)
>> > +           if (iter->idx == 0)
>> >                     return p;
>> >             (*pos)--;
>> >             iter->idx--;
>>
>>
>> Hi Roel,
>>
>> I'm not sure this is the right fix.
>> If you look at t_next, if there is no more page to look at,
>> iter_idx takes -1.
>>
>> A 0 value would mean: we are in the first index on the page, which means
>> there is something to read and we don't want to return NULL.
>>
>> I guess that would be better to turn idx into a signed int.
>
> Correct. This bug was added by:
>
>  50cdaf08a8ec1d7f43987705da7aff7cf949708f
>  ftrace: improve seq_operation of ftrace
>
> So the correct fix is to turn it into a signed int.
>
> Thanks,
>
> -- Steve
>

Both iter->idx and pg->index should be signed int I guess.

Roel is perhaps right, I don't know how is caught an unsigned long
when compared with a signed int if
this int is less than 0.
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