Received-SPF: pass (google.com: domain of linux-kernel+bounces-197449-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Date: Fri, 31 May 2024 13:36:35 -0700
From: Gatlin Newhouse <gatlin.newhouse@gmail.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, 
	Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	"H. Peter Anvin" <hpa@zytor.com>, Kees Cook <keescook@chromium.org>, 
	Marco Elver <elver@google.com>, Andrey Konovalov <andreyknvl@gmail.com>, 
	Andrey Ryabinin <ryabinin.a.a@gmail.com>, Nathan Chancellor <nathan@kernel.org>, 
	Nick Desaulniers <ndesaulniers@google.com>, Bill Wendling <morbo@google.com>, 
	Justin Stitt <justinstitt@google.com>, Andrew Morton <akpm@linux-foundation.org>, 
	Baoquan He <bhe@redhat.com>, Rick Edgecombe <rick.p.edgecombe@intel.com>, 
	Changbin Du <changbin.du@huawei.com>, Pengfei Xu <pengfei.xu@intel.com>, 
	Josh Poimboeuf <jpoimboe@kernel.org>, Xin Li <xin3.li@intel.com>, Jason Gunthorpe <jgg@ziepe.ca>, 
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com, 
	linux-hardening@vger.kernel.org, llvm@lists.linux.dev
Subject: Re: [PATCH] x86/traps: Enable UBSAN traps on x86
Message-ID: <5yx5aykf77x2gufpaf4nlrdhiqh6ioiqicazp4wq6dosu6d62g@xmj62qw7xa7q>
References: <20240529022043.3661757-1-gatlin.newhouse@gmail.com>
 <c068193b-75fb-49d2-9104-775051ffd941@citrix.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <c068193b-75fb-49d2-9104-775051ffd941@citrix.com>

On Thu, May 30, 2024 at 01:24:56AM UTC, Andrew Cooper wrote:
> On 29/05/2024 3:20 am, Gatlin Newhouse wrote:
> > diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> > index a3ec87d198ac..e3fbed9073f8 100644
> > --- a/arch/x86/include/asm/bug.h
> > +++ b/arch/x86/include/asm/bug.h
> > @@ -13,6 +13,14 @@
> >  #define INSN_UD2	0x0b0f
> >  #define LEN_UD2		2
> >  
> > +/*
> > + * In clang we have UD1s reporting UBSAN failures on X86, 64 and 32bit.
> > + */
> > +#define INSN_UD1	0xb90f
> > +#define LEN_UD1		2
> > +#define INSN_REX	0x67
> > +#define LEN_REX		1
> 
> That's an address size override prefix, not a REX prefix.

Good to know, thanks.

> What information is actually encoded in this UD1 instruction?? I can't
> find anything any documentation which actually discusses how the ModRM
> byte is encoded.

lib/ubsan.h has a comment before the ubsan_checks enum which links to line 113
in LLVM's clang/lib/CodeGen/CodeGenFunction.h which defines the values for the
ModRM byte. I think the Undefined Behavior Sanitizer pass does the actual
encoding of UB type to values but I'm not an expert in LLVM.

> ~Andrew