On Wed, Jul 01, 2026 at 12:58:12AM -0700, H. Peter Anvin wrote: > On July 1, 2026 12:48:34 AM PDT, Peter Zijlstra <[email protected]> wrote: > >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 > > The problem is that it being a single byte long it can appear in the middle > of another instruction.
I understand; this was in response to Dave's suggestion to make (supervisor) ENTER #UD, in which case we'd have to ensure no legit ENTERs exist.
