objtool: Split INSN_CONTEXT_SWITCH into INSN_SYSCALL and INSN_SYSRET

INSN_CONTEXT_SWITCH is ambiguous.  It can represent both call semantics
(SYSCALL, SYSENTER) and return semantics (SYSRET, IRET, RETS, RETU).
Those differ significantly: calls preserve control flow whereas returns
terminate it.

Objtool uses an arbitrary rule for INSN_CONTEXT_SWITCH that almost works
by accident: if in a function, keep going; otherwise stop.  It should
instead be based on the semantics of the underlying instruction.

In preparation for improving that, split INSN_CONTEXT_SWITCH into
INSN_SYCALL and INSN_SYSRET.

No functional change.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/19a76c74d2c051d3bc9a775823cafc65ad267a7a.1744095216.git.jpoimboe@kernel.org

Author: jpoimboe

tools/objtool/arch/x86/decode.c

@@ -522,7 +522,7 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 			case INAT_PFX_REPNE:
 				if (modrm == 0xca)
 					/* eretu/erets */
-					insn->type = INSN_CONTEXT_SWITCH;
+					insn->type = INSN_SYSRET;
 				break;
 			default:
 				if (modrm == 0xca)
@@ -535,11 +535,15 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 
 			insn->type = INSN_JUMP_CONDITIONAL;
 
-		} else if (op2 == 0x05 || op2 == 0x07 || op2 == 0x34 ||
-			   op2 == 0x35) {
+		} else if (op2 == 0x05 || op2 == 0x34) {
 
-			/* sysenter, sysret */
-			insn->type = INSN_CONTEXT_SWITCH;
+			/* syscall, sysenter */
+			insn->type = INSN_SYSCALL;
+
+		} else if (op2 == 0x07 || op2 == 0x35) {
+
+			/* sysret, sysexit */
+			insn->type = INSN_SYSRET;
 
 		} else if (op2 == 0x0b || op2 == 0xb9) {
 
@@ -676,7 +680,7 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 
 	case 0xca: /* retf */
 	case 0xcb: /* retf */
-		insn->type = INSN_CONTEXT_SWITCH;
+		insn->type = INSN_SYSRET;
 		break;
 
 	case 0xe0: /* loopne */
@@ -721,7 +725,7 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 		} else if (modrm_reg == 5) {
 
 			/* jmpf */
-			insn->type = INSN_CONTEXT_SWITCH;
+			insn->type = INSN_SYSRET;
 
 		} else if (modrm_reg == 6) {
 

tools/objtool/check.c

@@ -3684,7 +3684,8 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 			break;
 
-		case INSN_CONTEXT_SWITCH:
+		case INSN_SYSCALL:
+		case INSN_SYSRET:
 			if (func) {
 				if (!next_insn || !next_insn->hint) {
 					WARN_INSN(insn, "unsupported instruction in callable function");
@@ -3886,7 +3887,8 @@ static int validate_unret(struct objtool_file *file, struct instruction *insn)
 			WARN_INSN(insn, "RET before UNTRAIN");
 			return 1;
 
-		case INSN_CONTEXT_SWITCH:
+		case INSN_SYSCALL:
+		case INSN_SYSRET:
 			if (insn_func(insn))
 				break;
 			return 0;

tools/objtool/include/objtool/arch.h

@@ -19,7 +19,8 @@ enum insn_type {
 	INSN_CALL,
 	INSN_CALL_DYNAMIC,
 	INSN_RETURN,
-	INSN_CONTEXT_SWITCH,
+	INSN_SYSCALL,
+	INSN_SYSRET,
 	INSN_BUG,
 	INSN_NOP,
 	INSN_STAC,