Re: [PATCH v2] mm/vmalloc: widen guard region to defeat ENTER-based stack pivot

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 01, 2026 at 09:27:47AM +0200, Peter Zijlstra wrote:
> On Mon, Jun 29, 2026 at 03:29:50PM -0700, Dave Hansen wrote:
> 
> > But, really, if ENTER is so evil and nobody uses it, shouldn't we just
> > have an MSR bit somewhere to tell the CPU to #UD for it rather than
> > playing these stack games?
> 
> For supervisor mode only, I suppose. We can't ever get rid of userspace
> ENTER because legacy I suppose. But we can make sure the kernel is
> clean.
> 
> So yeah, having a knob to make supervisor-ENTER trap would be useful I
> suppose.

x86_64-defconfig builds clean with the below :-)


diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 1b387d5a195b..9e53db863203 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -642,6 +642,10 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 
 		break;
 
+	case 0xc8:
+		WARN("ENTER instruction at %s:%lx", sec->name, offset);
+		break;
+
 	case 0xc9:
 		/*
 		 * leave