Gilles Chanteperdrix wrote:
> Philippe Gerum wrote:
>  > Jan Kiszka wrote:
>  > > Philippe,
>  > > 
>  > > you recently said there is a bug in the x86_64 support when syscall
>  > > tracing is enabled. Now I think I stepped on it as well: In order to
>  > > validate my APIC frequency patches for that arch, I wanted to use LTTng
>  > > there. But as soon as I start the trace, the latency test fails to run,
>  > > prematurely exiting due to a segfault.
>  > 
>  > Exactly what Gilles sees on his box too, latency segfaulting at startup.
>  > On mine, the kernel does not even boot.
>  > 
>  >  Gdb and the kernel say that user
>  > > land jumped to address 0, I just yet failed to find out where they come
>  > > from. I strongly assume LTTng enables syscall tracing, because its
>  > > entry/exit instrumentations are inside the hook function
>  > > (syscall_trace_entry/leave).
>  > > 
>  > > Do you have any further details on your tracing issue? Does may
>  > > observation correlates with yours?
>  > 
>  > Quite frankly, I did not dig the issue that far yet, but yes, my first
>  > impression is that something is broken in the syscall return path (or
>  > entry?), and it shows when the return path to user-space is diverted in
>  > some way, either for security auditing, or likely for tracing like
>  > you've just reported.
> From what I have read in some comments, the syscall auditing function
> kmallocs some memory that is kfreed on syscall return. Obviously, this
> can not work with Xenomai.

Just a short update on this: Right before going mad over this bug, I
recalled some posting on ltt-dev by Mathieu Desnoyers about x86_64 and
some syscall tracing race. With this patch [1] applied, things work
again as they should! Then I followed his thread on LKML and tried the
second version of the patch [2], but that one does not work for us. Now
I wonder (but didn't analyse yet) if the first patch just moves some
race window around or actually fixes the bug for us?



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