objtool: Handle Clang RSP musical chairs

For no apparent reason (possibly related to CONFIG_KMSAN), Clang can
randomly pass the value of RSP to other registers and then back again to
RSP.  Handle that accordingly.

Fixes the following warnings:

  drivers/input/misc/uinput.o: warning: objtool: uinput_str_to_user+0x165: undefined stack state
  drivers/input/misc/uinput.o: warning: objtool: uinput_str_to_user+0x165: unknown CFA base reg -1

Reported-by: Arnd Bergmann <arnd@arndb.de>
Closes: https://lore.kernel.org/90956545-2066-46e3-b547-10c884582eb0@app.fastmail.com
Link: https://patch.msgid.link/240e6a172cc73292499334a3724d02ccb3247fc7.1772818491.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

Author: jpoimboe

tools/objtool/arch/x86/decode.c

@@ -395,52 +395,36 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 		if (!rex_w)
 			break;
 
-		if (modrm_reg == CFI_SP) {
-
-			if (mod_is_reg()) {
-				/* mov %rsp, reg */
-				ADD_OP(op) {
-					op->src.type = OP_SRC_REG;
-					op->src.reg = CFI_SP;
-					op->dest.type = OP_DEST_REG;
-					op->dest.reg = modrm_rm;
-				}
-				break;
-
-			} else {
-				/* skip RIP relative displacement */
-				if (is_RIP())
-					break;
-
-				/* skip nontrivial SIB */
-				if (have_SIB()) {
-					modrm_rm = sib_base;
-					if (sib_index != CFI_SP)
-						break;
-				}
-
-				/* mov %rsp, disp(%reg) */
-				ADD_OP(op) {
-					op->src.type = OP_SRC_REG;
-					op->src.reg = CFI_SP;
-					op->dest.type = OP_DEST_REG_INDIRECT;
-					op->dest.reg = modrm_rm;
-					op->dest.offset = ins.displacement.value;
-				}
-				break;
+		if (mod_is_reg()) {
+			/* mov reg, reg */
+			ADD_OP(op) {
+				op->src.type = OP_SRC_REG;
+				op->src.reg = modrm_reg;
+				op->dest.type = OP_DEST_REG;
+				op->dest.reg = modrm_rm;
 			}
-
 			break;
 		}
 
-		if (rm_is_reg(CFI_SP)) {
+		/* skip RIP relative displacement */
+		if (is_RIP())
+			break;
 
-			/* mov reg, %rsp */
+		/* skip nontrivial SIB */
+		if (have_SIB()) {
+			modrm_rm = sib_base;
+			if (sib_index != CFI_SP)
+				break;
+		}
+
+		/* mov %rsp, disp(%reg) */
+		if (modrm_reg == CFI_SP) {
 			ADD_OP(op) {
 				op->src.type = OP_SRC_REG;
-				op->src.reg = modrm_reg;
-				op->dest.type = OP_DEST_REG;
-				op->dest.reg = CFI_SP;
+				op->src.reg = CFI_SP;
+				op->dest.type = OP_DEST_REG_INDIRECT;
+				op->dest.reg = modrm_rm;
+				op->dest.offset = ins.displacement.value;
 			}
 			break;
 		}

tools/objtool/check.c

@@ -3000,6 +3000,20 @@ static int update_cfi_state(struct instruction *insn,
 				cfi->stack_size += 8;
 			}
 
+			else if (cfi->vals[op->src.reg].base == CFI_CFA) {
+				/*
+				 * Clang RSP musical chairs:
+				 *
+				 *   mov %rsp, %rdx [handled above]
+				 *   ...
+				 *   mov %rdx, %rbx [handled here]
+				 *   ...
+				 *   mov %rbx, %rsp [handled above]
+				 */
+				cfi->vals[op->dest.reg].base = CFI_CFA;
+				cfi->vals[op->dest.reg].offset = cfi->vals[op->src.reg].offset;
+			}
+
 
 			break;