linux-kernel - Re: [PATCH] tracing: Use linker magic instead of recasting ftrace_ops_list

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Date:   Thu, 18 Jun 2020 19:58:50 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Oscar Carter <oscar.carter@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] tracing: Use linker magic instead of recasting ftrace_ops_list_func()

On Thu, Jun 18, 2020 at 6:42 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Thu, 18 Jun 2020 01:12:37 +0200
> Jann Horn <jannh@...gle.com> wrote:
>
> > static ftrace_func_t ftrace_ops_get_list_func(struct ftrace_ops *ops)
> > +static ftrace_asm_func_t ftrace_ops_get_list_func(struct ftrace_ops *ops)
> >  {
> > +#if FTRACE_FORCE_LIST_FUNC
> > +       return ftrace_ops_list_func;
> > +#else
> >         /*
> >          * If this is a dynamic, RCU, or per CPU ops, or we force list func,
> >          * then it needs to call the list anyway.
> >          */
> > -       if (ops->flags & (FTRACE_OPS_FL_DYNAMIC | FTRACE_OPS_FL_RCU) ||
> > -           FTRACE_FORCE_LIST_FUNC)
> > +       if (ops->flags & (FTRACE_OPS_FL_DYNAMIC | FTRACE_OPS_FL_RCU))
> >                 return ftrace_ops_list_func;
> >
> >         return ftrace_ops_get_func(ops);
>
> But ftrace_ops_get_func() returns ftrace_func_t type, wont this complain?

No, because we only compile this case under FTRACE_FORCE_LIST_FUNC==0,
which means ARCH_SUPPORTS_FTRACE_OPS, which means the preprocessor
turns all occurrences of ftrace_asm_func_t into ftrace_func_t.

Essentially my idea here is to take the high-level rule "you can only
directly call ftrace_func_t-typed functions from assembly if
ARCH_SUPPORTS_FTRACE_OPS", and encode it in the type system. And then
the compiler won't complain as long as we make sure that we never cast
between the two types under ARCH_SUPPORTS_FTRACE_OPS==0.