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SubjectRe: objtool warning for next-20221118
On Wed, Nov 23, 2022 at 02:32:14PM -0800, Josh Poimboeuf wrote:
> On Wed, Nov 23, 2022 at 11:12:42AM -0800, Paul E. McKenney wrote:
> > On Wed, Nov 23, 2022 at 10:19:41AM -0800, Josh Poimboeuf wrote:
> > > On Wed, Nov 23, 2022 at 09:49:51AM -0800, Paul E. McKenney wrote:
> > > > > > > Perhaps the best way would be to stick a REACHABLE annotation in
> > > > > > > arch_cpu_idle_dead() or something?
> > > > > >
> > > > > > When I apply this on -next, I still get the objtool complaint.
> > > > > > Is there something else I should also be doing?
> > > > >
> > > > > Silly GCC is folding the inline asm. This works (but still doesn't seem
> > > > > like the right approach):
> > > > >
> > > > > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> > > > > index 26e8f57c75ad..128e7d78fedf 100644
> > > > > --- a/arch/x86/kernel/process.c
> > > > > +++ b/arch/x86/kernel/process.c
> > > > > @@ -702,7 +702,7 @@ static void (*x86_idle)(void);
> > > > > #ifndef CONFIG_SMP
> > > > > static inline void play_dead(void)
> > > > > {
> > > > > - BUG();
> > > > > + _BUG_FLAGS(ASM_UD2, 0, ASM_REACHABLE);
> > > > > }
> > > > > #endif
> > > >
> > > > I tried this, and still get:
> > > >
> > > > vmlinux.o: warning: objtool: do_idle+0x156: unreachable instruction
> > > >
> > > > Maybe my gcc is haunted?
> > >
> > > Weird, it worked for me. I have
> > >
> > > gcc version 12.2.1 20220819 (Red Hat 12.2.1-2) (GCC)
> >
> > Me, I have these, so quite a bit older:
> >
> > gcc version 8.5.0 20210514 (Red Hat 8.5.0-15) (GCC)
> > gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)
> >
> > > and I can't really fathom why that wouldn't work. Maybe it's a
> > > different issue? The "unreachable instruction" warning is limited to
> > > one, so when a first warning gets fixed, a second warning might suddenly
> > > become visible.
> > >
> > > Can you attach arch/x86/kernel/process.o?
> >
> > Attached!
>
> Hm, for whatever reason, that .o file is indeed missing the reachable
> annotation. <scratches head>

There are at least three definitions. Might I be getting the wrong one?

I have CONFIG_DEBUG_BUGVERBOSE=y and CONFIG_GENERIC_BUG=y, so I would
expect to be using the first one:

#define _BUG_FLAGS(ins, flags, extra) \
do { \
asm_inline volatile("1:\t" ins "\n" \
".pushsection __bug_table,\"aw\"\n" \
"2:\t" __BUG_REL(1b) "\t# bug_entry::bug_addr\n" \
"\t" __BUG_REL(%c0) "\t# bug_entry::file\n" \
"\t.word %c1" "\t# bug_entry::line\n" \
"\t.word %c2" "\t# bug_entry::flags\n" \
"\t.org 2b+%c3\n" \
".popsection\n" \
extra \
: : "i" (__FILE__), "i" (__LINE__), \
"i" (flags), \
"i" (sizeof(struct bug_entry))); \
} while (0)

> I confirmed the patch also fixes the warning with:
>
> gcc version 8.5.0 20210514 (Red Hat 8.5.0-10) (GCC)
>
> No idea why it's not working for you... but maybe it doesn't matter as
> I'm still thinking we should go with a different approach.

OK, then I will await your update.

Thanx, Paul

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