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

[Martin Schwidefsky]

On Tue, 25 Aug 2009 14:31:19 -0400
Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:

> * Frederic Weisbecker (fweisbec@xxxxxxxxx) wrote:
> > On Tue, Aug 25, 2009 at 06:59:14PM +0200, Frederic Weisbecker wrote:
> > > On Tue, Aug 25, 2009 at 12:20:04PM -0400, Mathieu Desnoyers wrote:
> > > > * 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.
> > > 
> > > 
> > > 
> > > 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 mean, I don't know how look like orig_eax at this stage.
> > 
> > Looking at arch/x86/kernel/process_32.c:copy_thread()
> > 
> > childregs = task_pt_regs(p);
> > *childregs = *regs;
> > childregs->ax = 0;
> > childregs->sp = sp;
> > 
> > p->thread.sp = (unsigned long) childregs;
> > p->thread.sp0 = (unsigned long) (childregs+1);
> > 
> > p->thread.ip = (unsigned long) ret_from_fork;
> > 
> > 
> > sp will be the struct pt_regs * passed to syscall_trace_leave()
> > later.
> > 
> > ax has the result of the fork syscall -> 0 for the child.
> > What about orig_eax which has the syscall nr? It depends on the pareent
> > and I don't know what it has at this stage.
> > 
> > I haven't seen crashes in x86 with kernel threads tracing, may be because
> > orig_eax is set to a valid syscall nr (may be even fork nr).
> > 
> > Perhaps it's not the case in s390 ?
> > 
> > Anyway, tracing kernel threads syscalls only gives us the fork return,
> > so it's something me may want to drop and trace higher level kernel
> > thread syscall wrappers instead.
> > 
> > Moreover every kernel threads is created through a kthreadd fork if
> > I'm not wrong, then it wouldn't be an accurate thing for us to trace the
> > fork calls in kernel thread. Tracing higher level kernel thread managment
> > sounds more interesting, we would then know who really created the thread,
> > etc...
> > 
> 
> (Well, I do not have time currently to look into the gory details
> (sorry), but let's try to take a step back from the problem.)
> 
> The design proposal for this kthread behavior wrt syscalls is based on a
> very specific and current kernel behavior, that may happen to change and
> that I have actually seen proven incorrect. For instance, some
> proprietary Linux driver does very odd things with system calls within
> kernel threads, like invoking them with int 0x80.

On s390 it is not allowed to execute the system call instruction svc
from kernel code to execute a system call function. You need to call
the system call function by name. The why is hidden in the critical
section cleanup in entry.S. There is a good reason why the inline
assemblies to execute an inline system call have been removed from
the kernel code.
 
> Yes, this is odd, but do we really want to tie the tracer that much to
> the actual OS implementation specificities ?
> 
> That sounds like a recipe for endless breakages and missing bits of
> instrumentation.
> 
> So my advice would be: if we want to trace the syscall entry/exit paths,
> let's trace them for the _whole_ system, and find ways to make it work
> for corner-cases rather than finding clever ways to diminish
> instrumentation coverage.

I guess that the real reason for the crash is hidden in the initialization
of the pt_regs structure of the kernel thread.

> Given the ret from fork example happens to be the first event fired
> after the thread is created, we should be able to deal with this problem
> by initializing the thread structure used by syscall exit tracing to an
> initial "ret from fork" value.

That is my best guess as well.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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