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

Frederic Weisbecker
2009-08-26T12:32:41Z

On Wed, Aug 26, 2009 at 09:38:20AM +0200, Heiko Carstens wrote:
> On Wed, Aug 26, 2009 at 12:04:26AM +0200, Martin Schwidefsky wrote:
> > On Tue, 25 Aug 2009 14:31:19 -0400
> > Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca> wrote:
> > > 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.
>
> That's broken.. some proprietary drivers even change the system call table.
> Do you want to be able to deal with that as well?
>
> > > 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.
>
> On s390 the reason is that the scvnr in the pt_regs structure of the initial
> kernel thread is initialized to 0. svcnr contains the system call number
> and system call number 0 does not exist.
> That's why we have
>
> static inline long syscall_get_nr(struct task_struct *task,
> struct pt_regs *regs)
> {
> return regs->svcnr ? regs->svcnr : -1;
> }
>
> Now, if you fork a kernel thread from the initial task the pt_regs structure
> gets copied. Upon ret_from_fork the trace exit path will get -1 for
> syscall_get_nr().
>
> > > 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.
>
> What would that value be? __NR_fork?
>
> Syscall tracing of kernel threads seems to be wrong. If somebody would do
> a "modprobe" and the init function of the module would create a kernel thread
> then syscall_get_nr() at the ret_from_fork path of the kernel thread would
> return __NR_init_module. That is of course only true if the old kernel_thread()
> API would be used. For kthread_create() it would return the syscall of the
> thread from which the kthread daemon was forked (the initial process I would
> guess, which was initialized to 0).
>
> So skipping kernel threads at the exit path seems so be the best fix, IMHO ;)

Yeah, we can decide to trace syscalls from kernel, but doing so through
the current syscalls tracepoints is broken.

> ---
> kernel/trace/trace_syscalls.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> Index: linux-next/kernel/trace/trace_syscalls.c
> ===================================================================
> --- linux-next.orig/kernel/trace/trace_syscalls.c
> +++ linux-next/kernel/trace/trace_syscalls.c
> @@ -253,6 +253,8 @@ void ftrace_syscall_exit(struct pt_regs
> struct ring_buffer_event *event;
> int syscall_nr;
>
> + if (!current->mm)
> + return;

Hendrik Brueckner already beat you at it and sent
a patch that ignores the TIF_SYSCALL_TRACEPOINT setting for
the kernel threads.

I'll add your acked by on it, thanks!

> syscall_nr = syscall_get_nr(current, regs);
> if (!test_bit(syscall_nr, enabled_exit_syscalls))
> return;