On Tue, Sep 15, 2020 at 10:02 AM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>
> On Tue, Sep 15, 2020 at 06:05:54PM +0200, Borislav Petkov wrote:
> > On Tue, Sep 15, 2020 at 10:18:16PM +0800, Rong Chen wrote:
> > > On Tue, Sep 15, 2020 at 03:55:19PM +0200, Borislav Petkov wrote:
> > > > On Tue, Sep 15, 2020 at 09:42:56PM +0800, kernel test robot wrote:
> > > > > tree/branch: https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git  x86/seves
> > > > > branch HEAD: e6eb15c9ba3165698488ae5c34920eea20eaa38e  KVM: nSVM: Avoid freeing uninitialized pointers in svm_set_nested_state()
> > > > >
> > > > > Warning in current branch:
> > > > >
> > > > > arch/x86/kernel/sev-es.o: warning: objtool: ist_exc_vmm_communication()+0x12: unreachable instruction
> > > > >
> > > > > Warning ids grouped by kconfigs:
> > > > >
> > > > > clang_recent_errors
> > > > > `-- x86_64-randconfig-r001-20200913
> > > > >     `-- arch-x86-kernel-sev-es.o:warning:objtool:ist_exc_vmm_communication():unreachable-instruction
> >
> > Ok, I can't trigger this with gcc-9, gcc-10 and clang-10.
> >
> > .config below says clang-12 so it could be some experimental
> > bug^Wfeature :)
> >
> > peterz says this has been fixed by:
> >
> > 318af7b80b6a ("Revert "kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled"")
> >
> > but that was reportedly for gcc.
> >
> > Lemme Cc the people of interest and leave the rest in here for
> > reference. Note that CONFIG_LIVEPATCH is enabled in that .config.
>
> If somebody can share the .o file, I can take a look.

Taking a look at the latest report of unreachable instruction from
objtool from 0day bot:
https://groups.google.com/g/clang-built-linux/c/IbF96FeMT8g

$ cd /tmp
$ git clone --depth=1
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git --branch
x86/seves
$ cd tip
$ gunzip <attached config.gz>
$ cp config .config
$ make LLVM=1 -j71 2> log.txt
$ grep warning: log.txt | cut -d ' ' -f 2- | sort | uniq -c
   3737 warning: objtool: asan.module_ctor()+0xc: call without frame
pointer save/setup
   3737 warning: objtool: asan.module_dtor()+0xc: call without frame
pointer save/setup
      1 warning: objtool: early_alloc()+0x6b: unreachable instruction
      1 warning: objtool: ist_exc_vmm_communication()+0x12:
unreachable instruction
      1 warning: objtool: kmem_cache_create_usercopy()+0x286:
unreachable instruction
      1 warning: objtool: oops_end()+0xc1: unreachable instruction
      1 warning: objtool: __register_nosave_region()+0x87: unreachable
instruction
$ ag ist_exc_vmm_communication
arch/x86/kernel/sev-es.c
1351:           ist_exc_vmm_communication(regs, error_code);
$ gzip -9 arch/x86/kernel/sev-es.o
<attached>

If I remove CONFIG_KASAN=y, I actually get way more unreachable
instruction failures!  Let me play with
tools/testing/ktest/config-bisect.pl and see if I can narrow down the
set of configs from the randconfig.  Also, it's possible that there's
more than one bug here.
-- 
Thanks,
~Nick Desaulniers