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Re: [PATCH -tip v5 07/10] kprobes/x86: Support kprobes jump optimization on x86
- From: Frederic Weisbecker <fweisbec at gmail dot com>
- To: Masami Hiramatsu <mhiramat at redhat dot com>
- Cc: Ingo Molnar <mingo at elte dot hu>, Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>, lkml <linux-kernel at vger dot kernel dot org>, systemtap <systemtap at sources dot redhat dot com>, DLE <dle-develop at lists dot sourceforge dot net>, Jim Keniston <jkenisto at us dot ibm dot com>, Srikar Dronamraju <srikar at linux dot vnet dot ibm dot com>, Christoph Hellwig <hch at infradead dot org>, Steven Rostedt <rostedt at goodmis dot org>, "H. Peter Anvin" <hpa at zytor dot com>, Anders Kaseorg <andersk at ksplice dot com>, Tim Abbott <tabbott at ksplice dot com>, Andi Kleen <andi at firstfloor dot org>, Jason Baron <jbaron at redhat dot com>, Mathieu Desnoyers <mathieu dot desnoyers at polymtl dot ca>
- Date: Tue, 24 Nov 2009 04:14:57 +0100
- Subject: Re: [PATCH -tip v5 07/10] kprobes/x86: Support kprobes jump optimization on x86
- References: <20091123232115.22071.71558.stgit@dhcp-100-2-132.bos.redhat.com> <20091123232211.22071.58974.stgit@dhcp-100-2-132.bos.redhat.com>
On Mon, Nov 23, 2009 at 06:22:11PM -0500, Masami Hiramatsu wrote:
> Introduce x86 arch-specific optimization code, which supports both of
> x86-32 and x86-64.
>
> This code also supports safety checking, which decodes whole of a function
> in which probe is inserted, and checks following conditions before
> optimization:
> - The optimized instructions which will be replaced by a jump instruction
> don't straddle the function boundary.
> - There is no indirect jump instruction, because it will jumps into
> the address range which is replaced by jump operand.
> - There is no jump/loop instruction which jumps into the address range
> which is replaced by jump operand.
> - Don't optimize kprobes if it is in functions into which fixup code will
> jumps.
>
> This uses stop_machine() for corss modifying code from int3 to jump.
> It doesn't allow us to modify code on NMI/SMI path. However, since
> kprobes itself doesn't support NMI/SMI code probing, it's not a
> problem.
>
> Changes in v5:
> - Introduce stop_machine-based jump replacing.
I realize now that int 3 live patching doesn't need stop_machine().
But still, I don't understand the int 3 unecessary step.
You first force int 3 patching, and later try to optimize
with a jump, using stop_machine().
But why the int 3 is a necessary first step? I guess it was
necessary first when you used it as a gate:
- patch with int 3, go to handler, go to old instruction
that was patched, jump to original code that folows
instruction that was patched
- set up detour buffer, execute handler (from int 3)
then route to detour buffer, and original code that
follows
- the code to be patched with the jump is now a
dead code, jump to it
And now that you use stop_machine(), the complexity could be
reduced to:
- decide kprobe mode
- if int 3, then do like usual
- if jmp, then prepare detour buffer, and patch with the jump,
without worrying about routing int 3 to the detour buffer
to create a dead code area. It is now safe because of stop_machine()
Of course it's possible I completely misunderstood the whole
thing :)