Messages in this thread | | | Date | Wed, 26 Aug 2009 14:59:43 +0200 | From | Heiko Carstens <> | Subject | Re: [PATCH 08/12] add trace events for each syscall entry/exit |
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On Wed, Aug 26, 2009 at 02:35:52PM +0200, Frederic Weisbecker wrote: > 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 :)
That won't always work as pointed out in the other example: - Process doing sys_init_module then scheduled away - User enables syscall tracing -> TIF_SYSCALL_FTRACE gets set - init function of the module gets called and is doing kernel_thread() (old API) -> kernel thread inherits TIF_SYSCALL_FTRACE.
I don't think that's what you want. You might want to clear the flag for new processes during fork (only for kernel threads I would guess).
At least the current patch leaves a hole.
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