[PATCH v2] ARM: support syscall tracing

[wade_farnsworth@mentor.com (Wade Farnsworth)]

Will Deacon wrote:
> 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.
>

I agree.  Looking at the other architectures, it seems the analogous 
function to ptrace_syscall_trace can return -1 under certain 
circumstances, but the original syscall value should be passed onto 
trace_sys_enter and returned from syscall_get_nr().  So, I'm thinking 
that we should modify our behavior accordingly.  What this means for us 
is that we never store -1 in the thread_info syscall field, and then 
pass that into trace_sys_enter instead of the ptrace_syscall_trace 
return value. Do you see any problems with this approach?

>> 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.
>

OK, I'll make that change.

Wade