Re: [PATCH] tracing: Use linker magic instead of recasting ftrace_ops_list_func()

[Steven Rostedt]

On Wed, 17 Jun 2020 23:30:07 +0200
Jann Horn <jannh@xxxxxxxxxx> wrote:
> [...]
> > +/* Defined by vmlinux.lds.h see the commment above arch_ftrace_ops_list_func for details */
> > +void ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip,
> > +                         struct ftrace_ops *op, struct pt_regs *regs);  
> [...]
> > +void arch_ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip)
> >  {  
> 
> Well, it's not like the function cast itself is the part that's
> problematic for CFI; the problematic part is when you actually make a
> C function call (in particular an indirect one) where the destination
> is compiled with a prototype that is different from the prototype used
> at the call site. Doing this linker hackery isn't really any better
> than shutting up the compiler warning by piling on enough casts or
> whatever. (There should be some combination of casts that'll shut up
> this warning, right?)

It's not called by C, it's called by assembly.

> 
> IIUC the real issue here is that ftrace_func_t is defined as a fixed
> type, but actually has different types depending on the architecture?
> If so, it might be cleaner to define ftrace_func_t differently
> depending on architecture, or something like that?

There's functions that use this type.

When you register a function to be used by the function tracer (that
will have 4 parameters). If the arch supports it, it will call it
directly from the trampoline in assembly, but if it does not, then the
C code will only let assembly call the two parameter version, that will
call the 4 parameter function (adding NULLs to the extra two arguments). 

> 
> And if that's not feasible, I think it would be better to at least
> replace this linker trickery with straightforward
> shut-up-the-compiler-casts - it'd be much easier to understand what's
> actually going on that way.

OK, what's the way to shut up the compiler for it, and we can have that
instead.

-- Steve