linux-kernel - Re: IBT related objtool warnings

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Message-ID: <20220407155106.GL2731@worktop.programming.kicks-ass.net>
Date:   Thu, 7 Apr 2022 17:51:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Poimboe, Josh" <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: IBT related objtool warnings

On Wed, Apr 06, 2022 at 10:43:33PM +0000, Edgecombe, Rick P wrote:
> Hi,
> 
> After commit:
> ed53a0d97192 x86/alternative: Use .ibt_endbr_seal to seal indirect calls
> 
> I am getting objtool "unreachable instruction" warnings.
> 
> On one system with gcc 8.4.0:
> vmlinux.o: warning: objtool: start_secondary()+0x10e: unreachable
> instruction
> 
> On another with gcc 11.2.1:
> vmlinux.o: warning: objtool: pvh_start_xen()+0x0: unreachable
> instruction
> 
> Let me know if any tests on my end would help.

This seems to make it go on gcc-11.2 tip/x86/urgent..

---

 arch/x86/entry/entry_64.S    | 3 +++
 arch/x86/platform/pvh/head.S | 1 +
 2 files changed, 4 insertions(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 4faac48ebec5..73d958522b6a 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -337,6 +337,9 @@ SYM_CODE_END(ret_from_fork)
 
 	call	\cfunc
 
+	/* For some configurations \cfunc ends up being a noreturn. */
+	REACHABLE
+
 	jmp	error_return
 .endm
 
diff --git a/arch/x86/platform/pvh/head.S b/arch/x86/platform/pvh/head.S
index 72c1e42d121d..7fe564eaf228 100644
--- a/arch/x86/platform/pvh/head.S
+++ b/arch/x86/platform/pvh/head.S
@@ -50,6 +50,7 @@
 #define PVH_DS_SEL		(PVH_GDT_ENTRY_DS * 8)
 
 SYM_CODE_START_LOCAL(pvh_start_xen)
+	UNWIND_HINT_EMPTY
 	cld
 
 	lgdt (_pa(gdt))