Messages in this thread | | | Date | Sun, 22 Feb 2009 21:33:33 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [PATCH 4/6] ftrace, x86: make kernel text writable only for conversions |
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* Steven Rostedt (rostedt@goodmis.org) wrote: > > > > On Sun, 22 Feb 2009, Andi Kleen wrote: > > > Steven Rostedt <rostedt@goodmis.org> writes: > > > > > From: Steven Rostedt <srostedt@redhat.com> > > > > > > Impact: keep kernel text read only > > > > > > Because dynamic ftrace converts the calls to mcount into and out of > > > nops at run time, we needed to always keep the kernel text writable. > > > > > > But this defeats the point of CONFIG_DEBUG_RODATA. This patch converts > > > the kernel code to writable before ftrace modifies the text, and converts > > > it back to read only afterward. > > > > > > The conversion is done via stop_machine and no IPIs may be executed > > > at that time. The kernel text is set to write just before calling > > > stop_machine and set to read only again right afterward. > > > > The very old text poke code I had for this just used a dynamic > > mapping elsewhere instead to modify the code. That's much less > > intrusive than changing the complete mappings. Any reason you can't use > > that too? > > > > We are changing over 19000 locations in the kernel. This touches almost > all kernel text pages anyway. You want to map a page in and out for over > 19000 locations? > > -- Steve >
Hi Steve,
Can you provide numbers to indicate why it's required to be so intrusive in the kernel mappings while doing these modifications ? I think opening such time window where standard code mapping is writeable globally in config RO_DATA kernels could open the door to unexpected side-effects, so ideally going through the "backdoor" page mapped by text_poke seems safer. Given similar performance, I would tend to use a text_poke-like approach.
Mathieu
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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