Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

[Josh Poimboeuf]

On Mon, Nov 26, 2018 at 05:02:17PM +0100, Peter Zijlstra wrote:
> On Mon, Nov 26, 2018 at 07:55:00AM -0600, Josh Poimboeuf wrote:
> > diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
> > index 8026d176f25c..d3869295b88d 100644
> > --- a/arch/x86/kernel/static_call.c
> > +++ b/arch/x86/kernel/static_call.c
> > @@ -9,13 +9,21 @@
> >  
> >  void static_call_bp_handler(void);
> >  void *bp_handler_dest;
> > +void *bp_handler_continue;
> >  
> >  asm(".pushsection .text, \"ax\"						\n"
> >      ".globl static_call_bp_handler					\n"
> >      ".type static_call_bp_handler, @function				\n"
> >      "static_call_bp_handler:						\n"
> > -    "ANNOTATE_RETPOLINE_SAFE						\n"
> > +#ifdef CONFIG_HAVE_STATIC_CALL_INLINE
> > +    ANNOTATE_RETPOLINE_SAFE
> > +    "call *bp_handler_dest						\n"
> > +    ANNOTATE_RETPOLINE_SAFE
> > +    "jmp *bp_handler_continue						\n"
> > +#else /* !CONFIG_HAVE_STATIC_CALL_INLINE */
> > +    ANNOTATE_RETPOLINE_SAFE
> >      "jmp *bp_handler_dest						\n"
> > +#endif
> >      ".popsection							\n");
> >  
> >  void arch_static_call_transform(void *site, void *tramp, void *func)
> > @@ -25,7 +33,10 @@ void arch_static_call_transform(void *site, void *tramp, void *func)
> >  	unsigned char insn_opcode;
> >  	unsigned char opcodes[CALL_INSN_SIZE];
> >  
> > -	insn = (unsigned long)tramp;
> > +	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE))
> > +		insn = (unsigned long)site;
> > +	else
> > +		insn = (unsigned long)tramp;
> >  
> >  	mutex_lock(&text_mutex);
> >  
> > @@ -41,8 +52,10 @@ void arch_static_call_transform(void *site, void *tramp, void *func)
> >  	opcodes[0] = insn_opcode;
> >  	memcpy(&opcodes[1], &dest_relative, CALL_INSN_SIZE - 1);
> >  
> > -	/* Set up the variable for the breakpoint handler: */
> > +	/* Set up the variables for the breakpoint handler: */
> >  	bp_handler_dest = func;
> > +	if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE))
> > +		bp_handler_continue = (void *)(insn + CALL_INSN_SIZE);
> >  
> >  	/* Patch the call site: */
> >  	text_poke_bp((void *)insn, opcodes, CALL_INSN_SIZE,
> 
> OK, so this is where that static_call_bp_handler comes from; you need
> that CALL to frob the stack.
> 
> But I still think it is broken; consider:
> 
> 	CPU0				CPU1
> 
> 	bp_handler = ponies;
> 
> 	text_poke_bp(, &static_call_bp_handler)
> 	  text_poke(&int3);
> 	  on_each_cpu(sync)
> 	  				<IPI>
> 					  ...
> 					</IPI>
> 
> 	  text_poke(/* all but first bytes */)
> 	  on_each_cpu(sync)
> 	  				<IPI>
> 					  ...
> 					</IPI>
> 
> 	  				<int3>
> 					  pt_regs->ip = &static_call_bp_handler
> 					</int3>
> 
> 					// VCPU takes a nap...
> 	  text_poke(/* first byte */)
> 	  on_each_cpu(sync)
> 	  				<IPI>
> 					  ...
> 					</IPI>
> 
> 					// VCPU sleeps more
> 	bp_handler = unicorn;
> 
> 					CALL unicorn
> 
> *whoops*
> 
> Now, granted, that is all rather 'unlikely', but that never stopped
> Murphy.

Good find, thanks Peter.

As we discussed on IRC, we'll need to fix this from within the int3
exception handler by faking the call: putting a fake return address on
the stack (pointing to right after the call) and setting regs->ip to the
called function.

And for the out-of-line case we can just jump straight to the function,
so the function itself will be the text_poke_bp() "handler".

So the static_call_bp_handler() trampoline will go away.

-- 
Josh