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From: Peter Zijlstra <peterz@infradead.org>
To: Randy Dunlap <rdunlap@infradead.org>
Cc: akpm@linux-foundation.org, broonie@kernel.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, linux-next@vger.kernel.org, mhocko@suse.cz,
	mm-commits@vger.kernel.org, sfr@canb.auug.org.au,
	Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: mmotm 2020-05-05-15-28 uploaded (objtool warning)
Date: Fri, 8 May 2020 12:38:30 +0200	[thread overview]
Message-ID: <20200508103830.GZ5298@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <36dc367a-f647-4ee8-a327-d1c3457a7940@infradead.org>

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

  parent reply	other threads:[~2020-05-08 10:38 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-05 22:29 mmotm 2020-05-05-15-28 uploaded akpm
     [not found] ` <36dc367a-f647-4ee8-a327-d1c3457a7940@infradead.org>
2020-05-08 10:38   ` Peter Zijlstra [this message]
2020-05-08 16:01     ` mmotm 2020-05-05-15-28 uploaded (objtool warning) Josh Poimboeuf
2020-05-08 17:28     ` Randy Dunlap

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