Re: [PATCH] ftrace: fix task's invalid comm of <...> when big pid

[Wang Yu]

ä 18/3/28 äå11:44, Steven Rostedt åé:
> On Wed, 28 Mar 2018 11:35:22 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > On Wed, 28 Mar 2018 20:32:27 +0800
> > Wang Yu <yuwang@xxxxxxxxxxxxxxxxx> wrote:
> >
> > > when pid is bigger than PID_MAX_DEFAULT, the comm of task
> > > is <...>, it is better use pid_max to compare
> > >
> > > Signed-off-by: Wang Yu <yuwang@xxxxxxxxxxxxxxxxx>
> > > ---
> > >   kernel/trace/trace.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >   mode change 100644 => 100755 kernel/trace/trace.c
> > >
> > > diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> > > old mode 100644
> > > new mode 100755
> > > index 20a2300..0d4bc7a
> > > --- a/kernel/trace/trace.c
> > > +++ b/kernel/trace/trace.c
> > > @@ -1976,7 +1976,7 @@ static void __trace_find_cmdline(int pid, char comm[])
> > >   		return;
> > >   	}
> > >   
> > > -	if (pid > PID_MAX_DEFAULT) {
> > > +	if (pid > pid_max) {
> > Thanks! this probably should go to stable.
> I take that back. This patch can cause a buffer overflow access.
>
> If you looked at the line after this check, you would see:
>
> 	if (pid > PID_MAX_DEFAULT) {
> 		strcpy(comm, "<...>");
> 		return;
> 	}
>
> 	map = savedcmd->map_pid_to_cmdline[pid];
>
> And if you looked to see what map_pid_to_cmdline is:
>
> struct saved_cmdlines_buffer {
> 	unsigned map_pid_to_cmdline[PID_MAX_DEFAULT+1];
>
> Your patch will access memory past the end of that array, and cause a
> bug.
>
> If you want to support more than PID_MAX_DEFAULT, a lot more needs to
> change than this. And a change like this isn't going to go to stable.
>
> What you can do is make that map_pid_to_cmdline array bigger.
>
> -- Steve

I am sorry about it, and as the number of cpu cores increases, the current

PID_MAX_DEFAULT is too small, our online machines set the pid_max 65536 as default, so the task
pid number bigger than PID_MAX_DEFAULT can't show the real comm (only <...>), so i want to
ajust the PID_MAX_DEFAULT upto 4x, and what do you think?

  * This controls the default maximum pid allocated to a process
  */
-#define PID_MAX_DEFAULT (CONFIG_BASE_SMALL ? 0x1000 : 0x8000)
+#define PID_MAX_DEFAULT (CONFIG_BASE_SMALL ? 0x1000 : 0x20000)

>   
>
> > -- Steve
> >
> > >   		strcpy(comm, "<...>");
> > >   		return;
> > >   	}