On Tue, May 05, 2020 at 10:40:43PM -0700, Randy Dunlap wrote: > On 5/5/20 3:29 PM, akpm@xxxxxxxxxxxxxxxxxxxx wrote: > > The mm-of-the-moment snapshot 2020-05-05-15-28 has been uploaded to > > > > http://www.ozlabs.org/~akpm/mmotm/ > > > > mmotm-readme.txt says > > > > README for mm-of-the-moment: > > > > http://www.ozlabs.org/~akpm/mmotm/ > > > > This is a snapshot of my -mm patch queue. Uploaded at random hopefully > > more than once a week. > > > > > on x86_64: > > arch/x86/hyperv/hv_apic.o: warning: objtool: hv_apic_write()+0x25: alternative modifies stack Wheee... this seems to have cured it for me. --- Subject: objtool: Allow no-op CFI ops in alternatives From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Date: Fri May 8 12:34:33 CEST 2020 Randy reported a false-positive: "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: 7117f16bf460 ("objtool: Fix ORC vs alternatives") Reported-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> --- tools/objtool/check.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-) --- a/tools/objtool/check.c +++ b/tools/objtool/check.c @@ -2078,17 +2078,18 @@ static int handle_insn_ops(struct instru 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;
