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

Josh Poimboeuf
2018-11-26T17:10:44Z

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