linux-kernel - Re: Adding TIF_TRACE_KERNEL to x86

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Date:	Sat, 27 Oct 2007 21:00:31 +0200
From:	Andi Kleen <ak@....de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Adding TIF_TRACE_KERNEL to x86_64

On Fri, Oct 26, 2007 at 03:37:38PM -0400, Mathieu Desnoyers wrote:
> 
> 1 - process A enters in a syscall, TIF_KERNEL_TRACE is cleared
> 2 - we activate TIF_KERNEL_TRACE
> 3 - process A returns from syscall (with wrong top of stack ?) -> segfault.
> 
> Am I on the right track ?

Yes. The code was not designed to allow that. The syscall path
has been optimized to go as fast as possible.

> 
> Can this be a concern with TIF_SYSCALL_TRACE also ? (potential race in
> ptrace ?)

ptrace only changes state in stopped processes; and those are not hanging
in syscalls. So no there is no race in a standard kernel.

If you wanted to change it the right way would be probably to test
for SYSCALL_TRACE too in the work flags and handle it there

-Andi

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