[PATCH v2 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/arch/x86/include/cfi_regs.h |  3 +++
 tools/objtool/cfi.h                       |  2 ++
 tools/objtool/check.c                     | 21 +++++++++++++++++----
 3 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/tools/objtool/arch/x86/include/cfi_regs.h b/tools/objtool/arch/x86/include/cfi_regs.h
index 79bc517efba8..4e443b49d0d3 100644
--- a/tools/objtool/arch/x86/include/cfi_regs.h
+++ b/tools/objtool/arch/x86/include/cfi_regs.h
@@ -22,4 +22,7 @@
 #define CFI_RA			16
 #define CFI_NUM_REGS		17
 
+#define STACKFRAME_BP_OFFSET	-16
+#define STACKFRAME_RA_OFFSET	-8
+
 #endif /* _OBJTOOL_CFI_REGS_H */
diff --git a/tools/objtool/cfi.h b/tools/objtool/cfi.h
index c7c59c6a44ee..2691b6ce4fcd 100644
--- a/tools/objtool/cfi.h
+++ b/tools/objtool/cfi.h
@@ -35,4 +35,6 @@ struct cfi_state {
 	bool end;
 };
 
+#define STACKFRAME_SIZE	16
+
 #endif /* _OBJTOOL_CFI_H */
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 2df9f769412e..50b3a4504db1 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1668,12 +1668,24 @@ 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 stackframe_start,
+				int expected_offset)
+{
+	return reg->base == CFI_CFA &&
+	       reg->offset == stackframe_start + 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 && cfi->cfa.offset >= STACKFRAME_SIZE &&
+	    check_reg_frame_pos(&cfi->regs[CFI_BP],
+			        -cfi->cfa.offset + STACKFRAME_SIZE,
+			        STACKFRAME_BP_OFFSET) &&
+	    check_reg_frame_pos(&cfi->regs[CFI_RA],
+			        -cfi->cfa.offset + STACKFRAME_SIZE,
+			        STACKFRAME_RA_OFFSET))
 		return true;
 
 	if (cfi->drap && cfi->regs[CFI_BP].base == CFI_BP)
@@ -1802,8 +1814,9 @@ 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 + STACKFRAME_SIZE,
+						STACKFRAME_BP_OFFSET)) {
 
 				/* mov %rsp, %rbp */
 				cfa->base = op->dest.reg;
-- 
2.25.4