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.
I once got this path into qemu+gdb, but did not trapped a case where the kernel decides to mess things up and return to NULL. Anyway, this debugging was not fully reliable, and I will retry soon (once my target has finished installing a new, full-blown 64-bit distro). Beside this, I already tried to analyse the return path but found nothing obvious on first sight. Hmm, wait, if tracing is enabled and we return from a Xenomai-handled syscall, I guess everything could go wrong if we then run into syscall_trace_leave over non-root domains, right? Maybe I should check if this could/actually does happen. [This bug is annoying. I have a huge pile of new patches here, all just waiting to be tested, and then this... :-/] Jan
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