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Date:   Thu, 18 Aug 2022 14:39:27 -0700
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     x86@kernel.org
Cc:     linux-kernel@vger.kernel.org, Pengfei Xu <pengfei.xu@intel.com>,
        "Yang, Weijiang" <weijiang.yang@intel.com>,
        "Su, Heng" <heng.su@intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Paolo Bonzini <pbonzini@redhat.com>
Subject: [PATCH v1.1] x86/ibt, objtool: Add IBT_NOSEAL()
Message-ID: <20220818213927.e44fmxkoq4yj6ybn@treble>
References: <cover.1660837839.git.jpoimboe@kernel.org>
 <5b86c2e3fff1f1e1f3fda126c315819205eb9d20.1660837839.git.jpoimboe@kernel.org>
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Add a macro which prevents a function from getting sealed if there are
no compile-time references to it.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
v1.1:
- add empty IBT_NOSEAL for CONFIG_X86_KERNEL_IBT=n

 arch/x86/include/asm/ibt.h | 11 +++++++++++
 tools/objtool/check.c      |  3 ++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/ibt.h b/arch/x86/include/asm/ibt.h
index 689880eca9ba..9b08082a5d9f 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.ibt_endbr_noseal\n\t"	\
+	_ASM_PTR fname "\n\t"				\
+	".popsection\n\t"
+
 static inline __attribute_const__ u32 gen_endbr(void)
 {
 	u32 endbr;
@@ -84,6 +94,7 @@ extern __noendbr void ibt_restore(u64 save);
 #ifndef __ASSEMBLY__
 
 #define ASM_ENDBR
+#define IBT_NOSEAL(name)
 
 #define __noendbr
 
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0cec74da7ffe..91678252a9b6 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -4096,7 +4096,8 @@ static int validate_ibt(struct objtool_file *file)
 		 * These sections can reference text addresses, but not with
 		 * the intent to indirect branch to them.
 		 */
-		if (!strncmp(sec->name, ".discard", 8)			||
+		if ((!strncmp(sec->name, ".discard", 8) &&
+		     strcmp(sec->name, ".discard.ibt_endbr_noseal"))	||
 		    !strncmp(sec->name, ".debug", 6)			||
 		    !strcmp(sec->name, ".altinstructions")		||
 		    !strcmp(sec->name, ".ibt_endbr_seal")		||
-- 
2.37.2