Commit ab3852ab authored May 08, 2020 by Peter Zijlstra

objtool: Allow no-op CFI ops in alternatives



Randy reported a false-positive:

  arch/x86/hyperv/hv_apic.o: warning: objtool: hv_apic_write()+0x25: alternative modifies stack

What happens is that:

	alternative_io("movl %0, %P1", "xchgl %0, %P1", X86_BUG_11AP,
 13d:   89 9d 00 d0 7f ff       mov    %ebx,-0x803000(%rbp)

decodes to an instruction with CFI-ops because it modifies RBP.
However, due to this being a !frame-pointer build, that should not in
fact change the CFI state.

So instead of dis-allowing any CFI-op, verify the op would've actually
changed the CFI state.

Fixes: 7117f16b ("objtool: Fix ORC vs alternatives")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>

parent cc1ac9c7

tools/objtool/check.c

+6 −5

Original line number	Diff line number	Diff line
		@@ -2060,17 +2060,18 @@ static int handle_insn_ops(struct instruction insn, struct insn_state state)
		struct stack_op *op;

		list_for_each_entry(op, &insn->stack_ops, list) {
		struct cfi_state old_cfi = state->cfi;
		int res;

		if (insn->alt_group) {
		WARN_FUNC("alternative modifies stack", insn->sec, insn->offset);
		return -1;
		}

		res = update_cfi_state(insn, &state->cfi, op);
		if (res)
		return res;

		if (insn->alt_group && memcmp(&state->cfi, &old_cfi, sizeof(struct cfi_state))) {
		WARN_FUNC("alternative modifies stack", insn->sec, insn->offset);
		return -1;
		}

		if (op->dest.type == OP_DEST_PUSHF) {
		if (!state->uaccess_stack) {
		state->uaccess_stack = 1;

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