On Tue, May 05, 2020 at 10:40:43PM -0700, Randy Dunlap wrote:
> On 5/5/20 3:29 PM, a...@linux-foundation.org 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 <pet...@infradead.org>
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 <rdun...@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
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;
