linux-kernel - Re: [PATCH v3 5/6] x86/ftrace: Use text

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Date:   Tue, 28 Jan 2020 20:17:06 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Kosina <jikos@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>, Miroslav Benes <mbenes@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Jessica Yu <jeyu@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, mhiramat@...nel.org,
        bristot@...hat.com, jbaron@...mai.com,
        torvalds@...ux-foundation.org, tglx@...utronix.de,
        mingo@...nel.org, namit@...are.com, hpa@...or.com, luto@...nel.org,
        ard.biesheuvel@...aro.org, live-patching@...r.kernel.org,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()

On Wed, Jan 29, 2020 at 01:46:55AM +0100, Jiri Kosina wrote:
> On Tue, 28 Jan 2020, Josh Poimboeuf wrote:
> 
> > > Anyway, I think that we might make your life easier with using the 
> > > proposed -Wsuggest-attribute=noreturn.
> > 
> > Maybe.  Though if I understand correctly, this doesn't help for any of 
> > the new warnings because they're for static functions, and this only 
> > warns about global functions.
> 
> Could you please provide a pointer where those have been 
> reported/analyzed?
> 
> For the cases I've seen so far, it has always been gcc deciding under 
> certain circumstances not to propagate __attribute__((__noreturn__)) from 
> callee to caller even in the cases when caller unconditionally called 
> callee.
> 
> AFAIU, the behavior is (and always will) be dependent on the state of gcc 
> optimizations, and therefore I don't see any other way than adding 
> __noreturn anotation transitively everywhere in order to silence objtool.
> 
> So those cases have to be fixed anyway.
> 
> What are the other cases please? Either I have completely missed those, or 
> they haven't been mentioned in this thread.

For example, see:

  https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/commit/?h=objtool-fixes&id=6265238af90b395a1e5e5032a41f012a552d8a9e

Many of those callees are static noreturns, for which we've *never*
needed annotations.  Disabling -fipa-pure-const has apparently changed
that.

-Wsuggest-attribute=noreturn doesn't seem to suggest annotations for
static functions, probably because most reasonable setups use -O2 which
allows GCC to detect them.

-- 
Josh