Re: [PATCH v5 00/18] Rework READ_ONCE() to improve codegen

[Peter Zijlstra]

On Thu, May 14, 2020 at 03:07:08PM +0200, Marco Elver wrote:
> On Thu, 14 May 2020 at 14:27, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, May 14, 2020 at 01:01:04PM +0100, Will Deacon wrote:
> >
> > > > +#if __has_feature(undefined_sanitizer)
> > >
> > > Hmm, this might want to be __has_feature(undefined_behavior_sanitizer)
> > > (and damn is that hard for a Brit to type out!)
> >
> > (I know right, it should be behaviour, dammit!)
> >
> > I tried without the condition, eg.:
> >
> > +#define __no_sanitize_undefined \
> > +               __attribute__((no_sanitize("undefined")))
> >
> > and it still generated UBSAN gunk.
> 
> Which ubsan calls are left? I'm trying to reproduce.

To be more precise, the patches were on top of:

  git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git entry-v5-the-rest

$ grep ubsan poke_int3_handler-clang10.asm
0074                    74: R_X86_64_PLT32      __ubsan_handle_load_invalid_value-0x4
0083                    83: R_X86_64_PLT32      __ubsan_handle_load_invalid_value-0x4
01c5                    1c5: R_X86_64_PLT32     __ubsan_handle_builtin_unreachable-0x4

I think the config was defconfig_x86-64 inspired with KASAN+UBSAN
enabled.

So I build with:

  touch arch/x86/kernel/alterantive.c;
  make CC=clang-10 V=1 O=defconfig-build/ arch/x86/kernel/alterantive.o

And then dump the output with:

  ./objdump-func.sh defconfig-build/arch/x86/kernel/alterantive.o poke_int3_handler

$ # cat objdump-func.sh
#!/bin/bash
objdump -dr $1 | awk "/^\$/ { P=0; } /$2[^>]*>:\$/ { P=1; O=strtonum(\"0x\" \$1); } { if (P) { o=strtonum(\"0x\" \$1); printf(\"%04x \", o-O); print \$0; } }"


Hope that is enough to reproduce.