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

Frederic Weisbecker
2009-08-26T13:31:37Z

On Wed, Aug 26, 2009 at 02:59:43PM +0200, Heiko Carstens wrote:
> 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.

Ah, there are callsites that use kernel_thread() directly?
Does it means that t->mm could be non NULL for such resulting
kernel threads, in that case it would be hard to hook on
do_fork() to check that.