Teach DTrace about BTI on arm64

The Branch Target Identification (BTI) Armv8-A extension adds new
instructions that can be placed where we may indirrectly branch to,
e.g. at the start of a function called via a function pointer. We can't
emulate these in DTrace as the kernel will have raised a different
exception before the DTrace handler has run.

Skip over the BTI instruction if it's used as the first instruction in
a function.

Sponsored by:	The FreeBSD Foundation

sys/cddl/contrib/opensolaris/uts/common/sys/dtrace.h

@@ -2466,6 +2466,9 @@ extern void dtrace_helpers_destroy(proc_t *);
 #define	B_DATA_MASK	0x00ffffff
 #define	B_INSTR		0x14000000
 
+#define	BTI_MASK	0xffffff3f
+#define	BTI_INSTR	0xd503241f
+
 #define	NOP_INSTR	0xd503201f
 
 #define	RET_INSTR	0xd65f03c0

sys/cddl/dev/fbt/aarch64/fbt_isa.c

@@ -118,6 +118,14 @@ fbt_provide_module_function(linker_file_t lf, int symindx,
 	instr = (uint32_t *)(symval->value);
 	limit = (uint32_t *)(symval->value + symval->size);
 
+	/*
+	 * Ignore any bti instruction at the start of the function
+	 * we need to keep it there for any indirect branches calling
+	 * the function on Armv8.5+
+	 */
+	if ((*instr & BTI_MASK) == BTI_INSTR)
+		instr++;
+
 	/* Look for stp (pre-indexed) operation */
 	found = false;
 	/*