[PATCH v3 1/3] objtool: check: Fully validate the stack frame

[Julien Thierry <jthierry@redhat.com>]

A valid stack frame should contain both the return address and the
previous frame pointer value.

On x86, the return value is placed on the stack by the calling
instructions. On other architectures, the callee need to explicitly
save the return address on the stack.

Add the necessary checks to verify a function properly sets up all the
elements of the stack frame.

Signed-off-by: Julien Thierry <jthierry@redhat.com>
---
 tools/objtool/check.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index f50ffa243c72..87f10e726a75 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1671,12 +1671,20 @@ static bool has_modified_stack_frame(struct instruction *insn, struct insn_state
 	return false;
 }
 
+static bool check_reg_frame_pos(const struct cfi_reg *reg,
+				int expected_offset)
+{
+	return reg->base == CFI_CFA &&
+	       reg->offset == expected_offset;
+}
+
 static bool has_valid_stack_frame(struct insn_state *state)
 {
 	struct cfi_state *cfi = &state->cfi;
 
-	if (cfi->cfa.base == CFI_BP && cfi->regs[CFI_BP].base == CFI_CFA &&
-	    cfi->regs[CFI_BP].offset == -16)
+	if (cfi->cfa.base == CFI_BP &&
+	    check_reg_frame_pos(&cfi->regs[CFI_BP], -cfi->cfa.offset) &&
+	    check_reg_frame_pos(&cfi->regs[CFI_RA], -cfi->cfa.offset + 8))
 		return true;
 
 	if (cfi->drap && cfi->regs[CFI_BP].base == CFI_BP)
@@ -1805,8 +1813,7 @@ static int update_cfi_state(struct instruction *insn, struct cfi_state *cfi,
 		case OP_SRC_REG:
 			if (op->src.reg == CFI_SP && op->dest.reg == CFI_BP &&
 			    cfa->base == CFI_SP &&
-			    regs[CFI_BP].base == CFI_CFA &&
-			    regs[CFI_BP].offset == -cfa->offset) {
+			    check_reg_frame_pos(&regs[CFI_BP], -cfa->offset)) {
 
 				/* mov %rsp, %rbp */
 				cfa->base = op->dest.reg;
-- 
2.25.4