Re: [PATCH 08/12] add trace events for each syscall entry/exit

Heiko Carstens
2009-08-26T13:00:44Z

On Wed, Aug 26, 2009 at 02:35:52PM +0200, Frederic Weisbecker wrote:
> On Tue, Aug 25, 2009 at 06:02:37PM +0200, Hendrik Brueckner wrote:
> > On Tue, Aug 25, 2009 at 04:15:49PM +0200, Frederic Weisbecker wrote:
> > > On Tue, Aug 25, 2009 at 02:50:27PM +0200, Hendrik Brueckner wrote:
> > > > There are at least two scenarios where syscall_get_nr() can return -1:
> > > >
> > > > 1. For example, ptrace stores an invalid syscall number, and thus,
> > > > tracing code resets it.
> > > > (see do_syscall_trace_enter in arch/s390/kernel/ptrace.c)
> > > >
> > > > 2. The syscall_regfunc() (kernel/tracepoint.c) sets the TIF_SYSCALL_FTRACE
> > > > (now: TIF_SYSCALL_TRACEPOINT) flag for all threads which includes
> > > > kernel threads.
> > > > However, the ftrace selftest triggers a kernel oops when testing syscall
> > > > trace points:
> > > > - The kernel thread is started as ususal (do_fork()),
> > > > - tracing code sets TIF_SYSCALL_FTRACE,
> > > > - the ret_from_fork() function is triggered and starts
> > > > ftrace_syscall_exit() with an invalid syscall number.
> > >
> > >
> > >
> > > I wonder if there is any way to identify such situation...?
> > For the second case, it might be an option to avoid setting the
> > TIF_SYSCALL_FTRACE flag for kernel threads.
> >
> > Kernel threads have task_struct->mm set to NULL.
> > (Thanks to Heiko for that hint ;-)
> >
> > The idea is then to check the mm field in syscall_regfunc() and
> > set the flag accordingly.
> >
> > However, I think the patch is an optional add-on becase checking
> > the syscall number is still required for case 1).
> >
> > ---
> > kernel/tracepoint.c | 4 +++-
> > 1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > --- a/kernel/tracepoint.c
> > +++ b/kernel/tracepoint.c
> > @@ -593,7 +593,9 @@ void syscall_regfunc(void)
> > if (!sys_tracepoint_refcount) {
> > read_lock_irqsave(&tasklist_lock, flags);
> > do_each_thread(g, t) {
> > - set_tsk_thread_flag(t, TIF_SYSCALL_FTRACE);
> > + /* Skip kernel threads. */
> > + if (t->mm)
> > + set_tsk_thread_flag(t, TIF_SYSCALL_FTRACE);
> > } while_each_thread(g, t);
> > read_unlock_irqrestore(&tasklist_lock, flags);
> > }
>
> Yeah, and as told before, syscalls tracing from kernel thread is
> an interesting point but we can't do it that way.
>
> I'm queuing this patch for .32, but I need you Signed-off-by to apply it :)

That won't always work as pointed out in the other example:
- Process doing sys_init_module then scheduled away
- User enables syscall tracing -> TIF_SYSCALL_FTRACE gets set
- init function of the module gets called and is doing kernel_thread()
(old API) -> kernel thread inherits TIF_SYSCALL_FTRACE.

I don't think that's what you want. You might want to clear the flag for
new processes during fork (only for kernel threads I would guess).

At least the current patch leaves a hole.