linux-kernel - Re: [PATCH 2/2] x86/unwind: Make CONFIG_UNWINDER

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Date:   Mon, 19 Mar 2018 21:28:03 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        torvalds@...ux-foundation.org, peterz@...radead.org, hpa@...or.com,
        tglx@...utronix.de
Subject: Re: [PATCH 2/2] x86/unwind: Make CONFIG_UNWINDER_ORC=y the default
 in kconfig for 64-bit

On Mon, Mar 19, 2018 at 04:22:55PM -0700, Matthias Kaehlcke wrote:
> > Here's a (surprisingly easy) fix for this particular issue, though I'd
> > be shocked if there weren't a bunch more issues lurking elsewhere.  Let
> > me know how it goes.
> 
> Thanks for having a look, this fixes the vast majority of warnings in
> a defconfig build!

Wow.  Consider me shocked :-)

> The remaining warnings are:
> 
> arch/x86/mm/pti.o: warning: objtool: pti_init() falls through to next
> function pti_user_pagetable_walk_pmd()
> s/debugfs/file.o: warning: objtool: full_proxy_llseek() falls through to next function full_proxy_read()
> fs/debugfs/file.o: warning: objtool: full_proxy_read() falls through to next function full_proxy_write()
> fs/debugfs/file.o: warning: objtool: full_proxy_write() falls through to next function full_proxy_poll()
> fs/debugfs/file.o: warning: objtool: full_proxy_poll() falls through to next function full_proxy_unlocked_ioctl()
> fs/debugfs/file.o: warning: objtool: full_proxy_unlocked_ioctl() falls through to next function fops_u8_open()

These all seem to be related to some weirdness with the UD2 instruction,
not necessarily an objtool issue per se.

Any chance this fixes some of the warnings?  If not, I can try to build
with clang and look a little deeper.

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index ab4711c63601..315a8757b565 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -124,7 +124,12 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 # define ASM_UNREACHABLE
 #endif
 #ifndef unreachable
-# define unreachable() do { annotate_reachable(); do { } while (1); } while (0)
+#define unreachable() \
+	do {					\
+		annotate_unreachable();		\
+		barrier_before_unreachable();	\
+		__builtin_unreachable();	\
+	} while (0)
 #endif
 
 /*