Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}

[Segher Boessenkool]

On Wed, Sep 14, 2022 at 11:21:00AM +0100, Josh Poimboeuf wrote:
> On Mon, Sep 12, 2022 at 06:31:14AM -0500, Segher Boessenkool wrote:
> > On Fri, Sep 09, 2022 at 11:07:04AM -0700, Josh Poimboeuf wrote:
> > > 2) Noreturn functions:
> > >    
> > >    There's no reliable way to determine which functions are designated
> > >    by the compiler to be noreturn (either explictly via function
> > >    attribute, or implicitly via a static function which is a wrapper
> > >    around a noreturn function.)
> > 
> > Or just a function that does not return for any other reason.
> > 
> > The compiler makes no difference between functions that have the
> > attribute and functions that do not.  There are good reasons to not
> > have the attribute on functions that do in fact not return.  The
> > not-returningness of the function may be just an implementation
> > accident, something you do not want part of the API, so it *should* not
> > have that attribute; or you may want the callers to a function to not be
> > optimised according to this knowledge (you cannot *prevent* that, the
> > compiler can figure it out it other ways, but still) for any other
> > reason.
> 
> Yes, many static functions that are wrappers around noreturn functions
> have this "implicit noreturn" property.

I meant functions that are noreturn intrinsically.  The trivial example:

void f(void)
{
	for (;;)
		;
}

>  I agree we would need to know
> about those functions (or, as Michael suggested, their call sites) as
> well.

Many "potentially does not return" functions (there are very many such
functions!) turn into "never returns" functions, for some inputs (or
something in the environment).  If the compiler specialises a code path
that does not return, you'll not see that marked up any way.  Of course
such a path should not be taken in the kernel, normally :-)

> > >    This information is needed because the
> > >    code after the call to such a function is optimized out as
> > >    unreachable and objtool has no way of knowing that.
> > 
> > Since June we (GCC) have -funreachable-traps.  This creates a trap insn
> > wherever control flow would otherwise go into limbo.
> 
> Ah, that's interesting, though I'm not sure if we'd be able to
> distinguish between "call doesn't return" traps and other traps or
> reasons for UD2.

The trap handler can see where the trap came from.  And then look up
that address in some tables or such.  Just like __bug_table?


Segher