Messages in this thread | | | Date | Tue, 24 Nov 2009 04:14:57 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH -tip v5 07/10] kprobes/x86: Support kprobes jump optimization on x86 |
| |
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 :)
| |