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Date: Thu, 30 May 2024 01:24:56 +0100
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Subject: Re: [PATCH] x86/traps: Enable UBSAN traps on x86
To: Gatlin Newhouse <gatlin.newhouse@gmail.com>,
 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
References: <20240529022043.3661757-1-gatlin.newhouse@gmail.com>
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From: Andrew Cooper <andrew.cooper3@citrix.com>
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In-Reply-To: <20240529022043.3661757-1-gatlin.newhouse@gmail.com>
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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.

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.

~Andrew