[PATCH v2] ARM: support syscall tracing

On Wed, Aug 15, 2012 at 05:58:44PM +0100, Wade Farnsworth wrote:

We need to set current_thread_info()->syscall, since it's used in the 
call to syscall_get_nr() in perf_syscall_{enter,exit}.

Damn. I think that also means we have a bug, given that the SYSCALL_TRACE
code can set this to -1, which gets used as an index into a bitmap by the
looks of it. Considering that we have to pass the syscall number to
trace_sys_enter anyway, it also seems broken.

What about moving the setting of ->syscall to 
syscall_trace_{enter,exit}?  That would preserve ptrace_syscall_trace() 
for the TIF_SYSCALL_TRACE case only, but ensure that the field is set 
the TRACEPOINT case as well.  Thoughts?

I'd be tempted to set the thing unconditionally before checking the thread
flag at the top of ptrace_syscall_trace. This hangs off the slowpath anyway
and it makes everything a lot more readable.

Will