[PATCH] objtool: add clang support

[Josh Poimboeuf]

Since the ORC unwinder was made the default on x86_64, clang-built
defconfig kernels have triggered some new objtool warnings:

  drivers/gpu/drm/i915/i915_gpu_error.o: warning: objtool: i915_error_printf()+0x6c: return with modified stack frame
  drivers/gpu/drm/i915/intel_display.o: warning: objtool: pipe_config_err()+0xa6: return with modified stack frame

The problem is that objtool has never seen clang-built binaries before.

Shockingly enough, objtool is apparently able to follow the code flow
mostly fine, except for one instruction sequence.  Instead of a LEAVE
instruction, clang restores RSP and RBP the long way:

   67c:   48 89 ec                mov    %rbp,%rsp
   67f:   5d                      pop    %rbp

Teach objtool about this new code sequence.

Reported-and-test-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 tools/objtool/check.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 92b6a2c21631..f02df714c18e 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -1386,6 +1386,17 @@ static int update_insn_state(struct instruction *insn, struct insn_state *state)
 				state->vals[op->dest.reg].offset = -state->stack_size;
 			}
 
+			else if (op->src.reg == CFI_BP && op->dest.reg == CFI_SP &&
+				 cfa->base == CFI_BP) {
+
+				/*
+				 * mov %rbp, %rsp
+				 *
+				 * Restore the original stack pointer (clang).
+				 */
+				state->stack_size = -state->regs[CFI_BP].offset;
+			}
+
 			else if (op->dest.reg == cfa->base) {
 
 				/* mov %reg, %rsp */
-- 
2.14.3