Messages in this thread | | | Date | Thu, 5 Aug 2004 20:35:30 +0530 | From | Suparna Bhattacharya <> | Subject | Re: [1/3] kprobes-func-args-268-rc3.patch |
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On Thu, Aug 05, 2004 at 03:51:12PM +0200, Andi Kleen wrote: > On Thu, Aug 05, 2004 at 07:03:48PM +0530, Suparna Bhattacharya wrote: > > On Thu, Aug 05, 2004 at 02:54:23PM +0200, Andi Kleen wrote: > > > On Thu, Aug 05, 2004 at 05:54:31PM +0530, Suparna Bhattacharya wrote: > > > > > I think you misunderstood Linus' suggestion. The problem with > > > > > modifying arguments on the stack frame is always there because the C > > > > > ABI allows it. One suggested solution was to use a second function > > > > > > > > I did realise that it is the ABI which allows this, but I thought > > > > that the only situation in which we know gcc to actually clobber > > > > arguments from the callee in practice is for tailcall optimization. > > > > > > It just breaks the most common workaround. > > > > Just curious, do you know if other cases/optimizations where the > > callee clobbers arguments on stack ? > > gcc can do it all the time. tail call is just a special case. > > It happens relatively rarely because often it caches the data > in registers. But when there is enough register pressure it can > be written back to the original argument slot. > > If you pass a structure by value it happens more often > (at least in older gccs, 3.5 now loads this into registers too) > > On -O0 code I would also expect it to happen often. > > > > > > > > > > I'm not sure if that can be guaranteed and yes saving bytes from > > > > stack would avoid the problem totally (hence the comment) and make > > > > it less tied to expected innards of the compiler. The only issue > > > > with that is deciding the maximum number of arguments so it is > > > > generic enough. > > > > > > 64bytes, aka 16 arguments seem far enough. > > > > OK, is there is consensus on this ? > > I think there is a clear consensus that anybody who uses 16 arguments > in a kernel function already did something very wrong. > > Passing structures by value may be reasonable, but not supporting > that for big structures is a reasonable restriction. > > > We'd have to make the code check for stack boundary etc and probably > > compare and copy back only if there has been a change. > > Why? And what stack boundary? > > The probe is not supposed to modify the arguments, isn't it, so I don't > see why you ever want to copy back. > > I would write the trampoline in assembly btw, doing such things in C is usually > very fragile.
Oh, I guess you are suggesting actually copying args forward on stack before passing control to the probe routine. Hence the assembly trampoline.
What I had in mind originally was a little different - i.e. just make the kprobes breakpoint handler itself save the additional 64 KB from stack the way it does for pt_regs right now, before iret ing into the jprobe handler function. And verify and restore that only if needed on the way back (i.e. second trap).
Regards Suparna
> > > > > > > > > > > > > > > Even with CONFIG_REGPARM, if you have a large > > > > number of arguments for example, is spill over into stack > > > > a possibility ? > > > > > > Yes. For more than three (Linux uses -mregparm=3) > > > Also varargs arguments will be always on the stack I think. > > > > Right, so making the copy dependent on !CONFIG_REGPARM wouldn't > > make sense would it ? > > Yes, it wouldn't. > > -Andi
-- Suparna Bhattacharya (suparna@in.ibm.com) Linux Technology Center IBM Software Lab, India
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