linux-kernel - Re: [PATCH v6 8/8] x86/kernel: jump

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Date:   Thu, 28 Dec 2017 11:39:15 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Arnd Bergmann <arnd@...db.de>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Will Deacon <will.deacon@....com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Thomas Garnier <thgarnie@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Russell King <linux@...linux.org.uk>,
        Paul Mackerras <paulus@...ba.org>,
        Catalin Marinas <catalin.marinas@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Petr Mladek <pmladek@...e.com>, Ingo Molnar <mingo@...hat.com>,
        James Morris <james.l.morris@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nicolas Pitre <nico@...aro.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-mips <linux-mips@...ux-mips.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        sparclinux@...r.kernel.org,
        "the arch/x86 maintainers" <x86@...nel.org>
Subject: Re: [PATCH v6 8/8] x86/kernel: jump_table: use relative references

On Thu, 28 Dec 2017 16:26:07 +0000
Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:

> On 28 December 2017 at 16:19, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Wed, 27 Dec 2017 08:50:33 +0000
> > Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> >  
> >>  static inline jump_label_t jump_entry_code(const struct jump_entry *entry)
> >>  {
> >> -     return entry->code;
> >> +     return (jump_label_t)&entry->code + entry->code;  
> >
> > I'm paranoid about doing arithmetic on abstract types. What happens in
> > the future if jump_label_t becomes a pointer? You will get a different
> > result.
> >  
> 
> In general, I share your concern. In this case, however, jump_label_t
> is typedef'd three lines up and is never used anywhere else.

I would agree if this was in a .c file, but it's in a header file,
which causes me to be more paranoid.

> 
> > Could we switch these calculations to something like:
> >
> >         return (jump_label_t)((long)&entrty->code + entry->code);
> >  
> 
> jump_label_t is local to this .h file, so it can be defined as u32 or
> u64 depending on the word size. I don't mind adding the extra cast,
> but I am not sure if your paranoia is justified in this particular
> case. Perhaps we should just use 'unsigned long' throughout?

Actually, that may be better. Have the return value be jump_label_t,
but the cast be "unsigned long". That way it should always work.

static inline jump_label_t jump_entry_code(...)
{
	return (unsigned long)&entry->code + entry->code;
}


-- Steve