Re: [RFC][PATCH 1/2] tracing/ftrace: syscall tracing infrastructure

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Oleg Nesterov (oleg@redhat.com) wrote:
> On 03/16, Mathieu Desnoyers wrote:
> >
> > utrace_add_engine()
> >   set_notify_resume(target);
> >
> > ok, so this is where the TIF_NOTIFY_RESUME thread flag is set. I notice
> > that it is set asynchronously with the execution of the target thread
> > (as I do with my TIF_KERNEL_TRACE thread flag).
> >
> > However, on x86_64, _TIF_DO_NOTIFY_MASK is only tested in
> > entry_64.S
> >
> > int_signal:
> > and
> > retint_signal:
> >
> > code paths. However, if there is no syscall tracing to do upon syscall
> > entry, the thread flags are not re-read at syscall exit and you will
> > miss the syscall exit returning from your target thread if this thread
> > was blocked while you set its TIF_NOTIFY_RESUME.
> 
> Afaics, TIF_NOTIFY_RESUME is not needed to trace syscall entry/exit.
> If engine wants the syscall tracing, utrace_set_events(UTRACE_SYSCALL_xxx)
> sets TIF_SYSCALL_TRACE. And syscall_trace_enter/syscall_trace_leave call
> tracehook_report_syscall_xxx().
> 
> Oleg.

I recall that TIF_SYSCALL_TRACE also suffers from the same problem as
TIF_NOTIFY_RESUME if set asynchronously with the target thread's
execution at least on x86_64 and arm. Do you take care to stop the
target thread in utrace_set_events ?

Mathieu

> 

-- 
Mathieu Desnoyers
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