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

Frederic Weisbecker
2009-08-25T16:59:18Z

On Tue, Aug 25, 2009 at 12:20:04PM -0400, Mathieu Desnoyers wrote:
> * Hendrik Brueckner (brueckner@linux.vnet.ibm.com) 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.

Yeah they do, but they don't use the sysenter path, they call the
syscall helpers directly, such as do_fork() or things like that.

The syscall tracepoints are set in the sysenter/sysexit path, then
it's no use to trace the kernel threads, it doesn't have any effect,
except random results in case of fork() calls, because we take
the ret_from_fork() path that also ends up to trace_sys_exit()
if the TIF_SYSCALL_TRACEPOINT thing is set, leading to such
asymetric tracing.

Kernel threads use syscalls toward wrappers such as create_thread().
So instead, statically defined tracepoints in create_thread() and such
other syscall wrappers for kernel threads seem more valuable, hmm?

> 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.

Well, I don't think that's the problem. The issue here, if I understand
correctly, is that kernel threads don't take the sysenter path, then never hit
the trace_sys_enter() call. And usually they won't ever hit any
trace_sys_exit() calls except in the fork() case, because we take
the ret_from_fork() path, which lead to syscall exit tracing due
to the TIF flags set.

At this stage, the syscall number is supposed to be stored in orig_eax,
but because the kernel thread hasn't called fork() through a syscall and
has called do_fork() directly, the regs values have nothing that look
like syscall parameters.

I guess we don't need to take the sys_enter tracing path to have a sane
orig_eax in the sys_exit tracing path (for non kernel threads).
Though I'm not sure about that, I should check to be sure.

> 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