Subject: Re: linux-next: build warnings after merge of the tip tree

From: Steven Rostedt <[email-masked]>
Date: Tue, 22 Mar 2022 12:17:29 -0400

On Tue, 22 Mar 2022 16:48:02 +0100
Peter Zijlstra <[email-masked]> wrote:

> On Tue, Mar 22, 2022 at 11:04:38AM -0400, Steven Rostedt wrote:
> 
> > > In recap:
> > > 
> > > 	__fentry__ -- push on trace-stack
> > > 	__ftail__  -- mark top-most entry complete
> > > 	__fexit__  -- mark top-most entry complete;
> > > 	              pop all completed entries  
> > 
> > Again, this would require that the tail-calls are also being traced.  
> 
> Which is why we should inhibit tail-calls if the function is notrace.
> 
> > > inhibit tail-calls to notrace.  
> > 
> > Just inhibiting tail-calls to notrace would work without any of the above.  
> 
> I'm lost again; what? Without any of the above you got nothing because
> return-trampoline will not work.

I think this got "lost in translation".

 "Inhibiting tail-calls to notrace"

Is a bit ambiguous because of the "to notrace" which would be different if
I had said "on notrace" which I may have screwed up the grammar here. Let
me be more precise.

 "Limiting tail-calls to only notrace functions"

That I think is a bit less ambiguous. English sucks.

> 
> > But my fear is that will cause a noticeable performance impact.  
> 
> Most code isn't in fact notrace, and call+ret aren't *that* expensive.

  "isn't in fact notrace" Ug! Double negatives!

This gets even more confusing when we are saying "notrace" which is a
negative. We should probably just say "traced" functions which makes
communication a bit more straight forward.

> 
> > > It's function graph tracing, kretprobes and whatever else this rethook
> > > stuff is about that needs this because return trampolines will stop
> > > working somewhere in the not too distant future.  
> > 
> > Another crazy solution is to have:
> > 
> > func_A:
> > 	call __fentry__
> > 	...
> > tail:	jmp 1f 
> > 	call 1f  
> 	
> > 	call __fexit__
> > 	ret
> > 1:	jmp func_B
> > 
> > 
> > where the compiler tells us about "tail:" and that we know that func_A ends
> > with a tail call, and if we want to trace the end of func_A we convert that
> > jmp 1f into a nop. And then we call the func_B and it's return comes back
> > to where we call __fexit__ and then return normally.  
> 
> At that point giving us something like:
> 
> 1:
> 	pushsection __ftail_loc
> 	.long	1b - .
> 	popsection
> 
> 	jmp.d32	func_B
> 	call	__fexit__
> 	ret
> 
> is smaller and simpler, we can patch the jmp.d32 to call when tracing.
> The only problem is SLS, that might wants an int3 after jmp too
> ( https://www.amd.com/en/corporate/product-security/bulletin/amd-sb-1026 ).
> 
> That does avoid the need for __ftail__ I suppose.

Which is basically what I said earlier ;-)

  https://lore.kernel.org/all/[email-masked]/

> Or maybe another solution is:
> 
> funcA:
> 	[..]
> 	jmp funcB
> 	call __fexit__
> 	ret
> 
> And if funcA is being traced, we change jmp to a call.
> 
> 	[..]
> 	call funcB
> 	call __fexit__
> 	ret
> 
> Such that we only remove the tail calls if we enable tracing on the
> function with the tail call.

-- Steve