Messages in this thread | | | Date | Tue, 25 Aug 2009 18:59:14 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 08/12] add trace events for each syscall entry/exit |
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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
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