x86/ibt, objtool: Add IBT_NOSEAL()

Add a macro which prevents a function from getting sealed if there are
no compile-time references to it.

Signed-off-by: Josh Poimboeuf <[email protected]>
Message-Id: <20220818213927.e44fmxkoq4yj6ybn@treble>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: jpoimboe

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
 

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")		||