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

Hendrik Brueckner
2009-08-25T16:02:29Z

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);
}