Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 17 Aug 2022 18:10:45 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     "Xu, Pengfei" <pengfei.xu@intel.com>,
        "Yang, Weijiang" <weijiang.yang@intel.com>,
        "Su, Heng" <heng.su@intel.com>, linux-kernel@vger.kernel.org,
        Josh Poimboeuf <jpoimboe@redhat.com>, pbonzini@redhat.com,
        x86@kernel.org
Subject: Re: [PATCH] x86/kvm, objtool: Avoid fastop ENDBR from being sealed
Message-ID: <20220818011045.v4baekgxajylqxvh@treble>
References: <PH0PR11MB4839B4D2FB8B8D8D52A62C7F9A629@PH0PR11MB4839.namprd11.prod.outlook.com>
 <YvzJTxOwmikAlZ6j@worktop.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <YvzJTxOwmikAlZ6j@worktop.programming.kicks-ass.net>
Precedence: bulk

On Wed, Aug 17, 2022 at 12:56:15PM +0200, Peter Zijlstra wrote:
> Add (yet another) annotation to inhibit objtool from sealing a specific
> ENDBR instance.

Nah, just add a throwaway reference to the function.  Then no objtool
change is needed:

diff --git a/arch/x86/include/asm/ibt.h b/arch/x86/include/asm/ibt.h
index 689880eca9ba..3ae795150374 100644
--- a/arch/x86/include/asm/ibt.h
+++ b/arch/x86/include/asm/ibt.h
@@ -31,6 +31,16 @@
 
 #define __noendbr	__attribute__((nocf_check))
 
+/*
+ * Create a dummy function pointer reference to prevent objtool from marking
+ * the function as needing to be "sealed" (i.e. ENDBR converted to NOP by
+ * apply_ibt_endbr()).
+ */
+#define IBT_NOSEAL(fname)				\
+	".pushsection .discard.endbr.noseal\n\t"	\
+	_ASM_PTR fname "\n\t"				\
+	".popsection\n\t"
+
 static inline __attribute_const__ u32 gen_endbr(void)
 {
 	u32 endbr;
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index f8382abe22ff..88623add0db4 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -316,7 +316,8 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
 	".align " __stringify(FASTOP_SIZE) " \n\t" \
 	".type " name ", @function \n\t" \
 	name ":\n\t" \
-	ASM_ENDBR
+	ASM_ENDBR \
+	IBT_NOSEAL(name)
 
 #define FOP_FUNC(name) \
 	__FOP_FUNC(#name)