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

[Mathieu Desnoyers]

* Hendrik Brueckner (brueckner@xxxxxxxxxxxxxxxxxx) 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);

Uh ? kernel threads can invoke a system call. There are rare places
where kernel code actually invoke system calls. I don't see why we
should not deal with them.

Moreover, the problem you face is more general: if we set the
TIF_SYSCALL_FTRACE flag of a standard thread right in the middle of its
system call, x86_64 will cause the syscall exit to execute by re-reading
the thread flags and run a syscall trace exit.

We could simply initialize the "saved system calls id" number to
something like -1, so that if we happen to return from a syscall that
did not get its id recorded at syscall entry, we know it because it's
not initialized.

We would need to carefully put back the -1 value after clearing the
thread flag when we stop tracing too (while still holding a mutex).

Mathieu

>  		} while_each_thread(g, t);
>  		read_unlock_irqrestore(&tasklist_lock, flags);
>  	}
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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