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

    Thanks.



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