[PATCH 5/5] objtool: Support stack-swizzle

[Peter Zijlstra]

Natively support the stack swizzle pattern:

	mov %rsp, (%[tos])
	mov %[tos], %rsp
	...
	pop %rsp

with the constraint that %[tos] must be !arch_callee_saved_reg().

It uses the (newly minted) scratch regs to link the first two
stack-ops, and detect the SP to SP_INDIRECT swizzle.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 tools/objtool/check.c |   43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1937,6 +1937,34 @@ static int update_cfi_state(struct instr
 					cfa->offset = -cfi->vals[op->src.reg].offset;
 					cfi->stack_size = cfa->offset;
 
+				} else if (cfa->base == CFI_SP &&
+					   cfi->regs[op->src.reg].base == CFI_SP_INDIRECT &&
+					   cfi->regs[op->src.reg].offset == cfa->offset) {
+
+					/*
+					 * Stack swizzle:
+					 *
+					 * 1: mov %rsp, (%[tos])
+					 * 2: mov %[tos], %rsp
+					 *    ...
+					 * 3: pop %rsp
+					 *
+					 * Where:
+					 *
+					 * 1 - places a pointer to the previous
+					 *     stack at the Top-of-Stack of the
+					 *     new stack.
+					 *
+					 * 2 - switches to the new stack.
+					 *
+					 * 3 - pops the Top-of-Stack to restore
+					 *     the original stack.
+					 *
+					 * Note:
+					 * %[tos] must not be a callee saved reg
+					 */
+					cfa->base = CFI_SP_INDIRECT;
+
 				} else {
 					cfa->base = CFI_UNDEFINED;
 					cfa->offset = 0;
@@ -2028,6 +2056,13 @@ static int update_cfi_state(struct instr
 
 		case OP_SRC_POP:
 		case OP_SRC_POPF:
+			if (op->dest.reg == CFI_SP && cfa->base == CFI_SP_INDIRECT) {
+
+				/* pop %rsp; # restore from a stack swizzle */
+				cfa->base = CFI_SP;
+				break;
+			}
+
 			if (!cfi->drap && op->dest.reg == cfa->base) {
 
 				/* pop %rbp */
@@ -2154,6 +2189,14 @@ static int update_cfi_state(struct instr
 			/* mov reg, disp(%rsp) */
 			save_reg(cfi, op->src.reg, CFI_CFA,
 				 op->dest.offset - cfi->cfa.offset);
+
+		} else if (op->src.reg == CFI_SP && op->dest.offset == 0) {
+
+			/* mov %rsp, (%reg); # setup a stack swizzle. */
+			if (!arch_callee_saved_reg(op->dest.reg)) {
+				__save_reg(cfi, op->dest.reg, CFI_SP_INDIRECT, cfi->cfa.offset);
+				skip_wipe = true;
+			}
 		}
 
 		break;