From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Date: Tue, 27 Feb 2007 20:24:27 +0000 Subject: Re: debug registers and fork Message-Id: <20070227122427.bf23af2f.akpm@linux-foundation.org> List-Id: References: <20070226235154.GB19630@frankl.hpl.hp.com> In-Reply-To: <20070226235154.GB19630@frankl.hpl.hp.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: eranian@hpl.hp.com Cc: linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org, ak@suse.de, tony.luck@intel.com, Roland McGrath > On Mon, 26 Feb 2007 15:51:54 -0800 Stephane Eranian wrote: > Hello, > > I have come across an issue with a monitoring using the > hardware debug registers on ia64/i386/x86-64. > > It seems that the way debug registers are inherited across fork > differs between ia-64 and i386/x86-64. On ia-64, the debug registers > are NEVER inherited in the child. The copy_thread() routine clears > the necessary thread flags to avoid reloading the debug registers in > the child. > > Now, on x86-64, it appears that the TIF_DEBUG flag is inherited via > setup_thread_stack(). By virtue of dup_task_struct() the debug registers > get copied into the child task on fork. So the child has active breakpoints, > unless I am mistaken somewhere. > > Given the way the ptrace() interface works, I would tend to > think that the ia-64 way is the correct one. Any comment? > > Furthermore, on i386/x86-64, when switching out from a task with TIF_DEBUG > enabled to another which does not, it seems we do not clear the debug > registers (at least dr7) so they become inactive. > Let's cc Roland - he's totally rewritten ptrace and probably knows this stuff.