Re: [RFC][PATCH v2] ftrace/x86: Emulate call function while updating in breakpoint handler

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 01 May 2019 10:26:32 +0200
Nicolai Stange <nstange@suse.de> wrote:

> > +extern asmlinkage void ftrace_emulate_call_irqon(void);
> > +extern asmlinkage void ftrace_emulate_call_irqoff(void);
> > +extern asmlinkage void ftrace_emulate_call_nmi(void);
> > +extern asmlinkage void ftrace_emulate_call_update_irqoff(void);
> > +extern asmlinkage void ftrace_emulate_call_update_irqon(void);
> > +extern asmlinkage void ftrace_emulate_call_update_nmi(void);
> > +
> > +static DEFINE_PER_CPU(void *, ftrace_bp_call_return);
> > +static DEFINE_PER_CPU(void *, ftrace_bp_call_nmi_return);  
> 
> Andy mentioned #DB and #MC exceptions here:
> https://lkml.kernel.org/r/C55DED25-C60D-4731-9A6B-92BDA8771766@amacapital.net
> 
> I think that #DB won't be possible, provided the trampolines below get
> tagged as NOKPROBE (do_int3() and ftrace_int3_handler() already have
> it).
> 
> It's highly theoretic, but tracing do_machine_check() could clobber
> ftrace_bp_call_return or ftrace_bp_call_nmi_return?

Probably shouldn't trace do_machine_check() then ;-)

> 
> 
> > +#ifdef CONFIG_SMP
> > +#ifdef CONFIG_X86_64
> > +# define BP_CALL_RETURN		"%gs:ftrace_bp_call_return"
> > +# define BP_CALL_NMI_RETURN	"%gs:ftrace_bp_call_nmi_return"
> > +#else
> > +# define BP_CALL_RETURN		"%fs:ftrace_bp_call_return"
> > +# define BP_CALL_NMI_RETURN	"%fs:ftrace_bp_call_nmi_return"
> > +#endif
> > +#else /* SMP */
> > +# define BP_CALL_RETURN		"ftrace_bp_call_return"
> > +# define BP_CALL_NMI_RETURN	"ftrace_bp_call_nmi_return"
> > +#endif
> > +
> > +/* To hold the ftrace_caller address to push on the stack */
> > +void *ftrace_caller_func = (void *)ftrace_caller;  
> 
> The live patching ftrace_ops need ftrace_regs_caller.

Ah, you're right. Luckily ftrace_regs_caller is a superset of
ftrace_caller. That is, those only needing ftrace_caller can do fine
with ftrace_regs_caller (but not vice versa).

Easy enough to fix.

> 
> 
> > +
> > +asm(
> > +	".text\n"
> > +
> > +	/* Trampoline for function update with interrupts enabled */
> > +	".global ftrace_emulate_call_irqoff\n"
> > +	".type ftrace_emulate_call_irqoff, @function\n"
> > +	"ftrace_emulate_call_irqoff:\n\t"
> > +		"push "BP_CALL_RETURN"\n\t"
> > +		"push ftrace_caller_func\n"
> > +		"sti\n\t"
> > +		"ret\n\t"
> > +	".size ftrace_emulate_call_irqoff, .-ftrace_emulate_call_irqoff\n"
> > +
> > +	/* Trampoline for function update with interrupts disabled*/
> > +	".global ftrace_emulate_call_irqon\n"  
> 
> The naming is perhaps a bit confusing, i.e. "update with interrupts
> disabled" vs. "irqon"... How about swapping irqoff<->irqon?

I just used the terminology Linus used. It is confusing. Perhaps just
call it ftrace_emulate_call (for non sti case) and
ftrace_emulate_call_sti for the sti case. That should remove the
confusion.

-- Steve