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From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>,
	kbuild test robot <fengguang.wu@intel.com>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f
Date: Wed, 7 Feb 2018 20:10:07 +0100	[thread overview]
Message-ID: <20180207191007.GD25235@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20180207190651.GB25201@hirez.programming.kicks-ass.net>

On Wed, Feb 07, 2018 at 08:06:51PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 07, 2018 at 11:01:29AM -0800, Linus Torvalds wrote:
> > On Wed, Feb 7, 2018 at 10:38 AM, Randy Dunlap <rdunlap@infradead.org> wrote:
> > > On 02/07/2018 10:13 AM, Linus Torvalds wrote:
> > >>
> > >> That said, intel only _documents_ UD2 (0f 0b).
> > >
> > > Intel Order Number: 325383-064US, October 2017, documents UD0, UD1, and UD2.
> > > Section A.2.5, Table A-1, says:
> > 
> > Ahh, I had an older version.
> > 
> > Looking at the latest one I can find (325462-065US), it does specify
> > that it has a modrm byte:
> > 
> >   0F FF /r   UD0 1 r32, r/m32
> > 
> > so I think that our opcode maps are wrong, and it's a bit dangerous to
> > put random constants right after the UD0.
> > 
> > Maybe we should make our use of UD0 have a third byte: add a harmless
> > modrm byte before the warning constants?
> > 
> > But yes, at least my objdump just thinks it's a bad 2-byte sequence,
> > and doesn't look at any modrm bytes at all.
> 
> Look what my objdump does:
> 
> $ objdump --version
> GNU objdump (GNU Binutils for Debian) 2.29.90.20180122
> 
> $ objdump -dr defconfig-build/kernel/sched/core.o | grep ud0
>     183a:       0f ff 65 48             ud0    0x48(%rbp),%esp
>     1881:       0f ff c3                ud0    %ebx,%eax
>     18b1:       0f ff 89 c0 48 0f a3    ud0    -0x5cf0b740(%rcx),%ecx
>     1940:       0f ff 89 c0 48 0f a3    ud0    -0x5cf0b740(%rcx),%ecx
>     19c2:       0f ff 89 c0 48 0f a3    ud0    -0x5cf0b740(%rcx),%ecx
> 
> So yeah, we're screwed :-(
> 
> Adding a harmless modr/m to our UD0 would grow the kernel image. ARGGH

PeterA, at the time there was talk of arranging a single byte UD
instruction. Any luck arranging that?

      reply	other threads:[~2018-02-07 19:10 UTC|newest]

Thread overview: 35+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-07 11:44 [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f kbuild test robot
2018-02-07 18:13 ` Linus Torvalds
2018-02-07 18:35   ` Borislav Petkov
2018-02-07 18:49     ` Peter Zijlstra
2018-02-07 19:03       ` Linus Torvalds
2018-02-07 19:14         ` Peter Zijlstra
2018-02-07 19:28           ` Borislav Petkov
2018-02-07 19:43             ` Linus Torvalds
2018-02-07 20:24               ` Borislav Petkov
2018-02-08  9:13               ` Peter Zijlstra
2018-02-08  9:35                 ` Peter Zijlstra
2018-02-08  9:46                 ` Borislav Petkov
2018-02-08  9:47                 ` David Laight
2018-02-08 10:13                   ` Peter Zijlstra
2018-02-08 17:27                 ` Linus Torvalds
2018-02-08 18:03                   ` Peter Zijlstra
2018-02-08 18:15                     ` Linus Torvalds
2018-02-08 19:44                       ` Peter Zijlstra
2018-02-08 20:02                         ` Linus Torvalds
2018-02-08 20:31                         ` Borislav Petkov
2018-02-08 23:09                           ` [PATCH 0/2] objtool fixes on top of Peter's WARN UD2 patch Josh Poimboeuf
2018-02-08 23:09                             ` [PATCH 1/2] objtool: Fix seg fault in ignore_unreachable_insn() Josh Poimboeuf
2018-02-13 11:29                               ` [tip:x86/pti] objtool: Fix segfault " tip-bot for Josh Poimboeuf
2018-02-15  0:26                               ` tip-bot for Josh Poimboeuf
2018-02-08 23:09                             ` [PATCH 2/2] x86: Annotate WARN-related UD2 as reachable Josh Poimboeuf
2018-02-13 11:30                               ` [tip:x86/pti] x86/debug, objtool: Annotate WARN()-related " tip-bot for Josh Poimboeuf
2018-02-15  0:26                               ` tip-bot for Josh Poimboeuf
2018-02-09  8:13                             ` [PATCH 0/2] objtool fixes on top of Peter's WARN UD2 patch Peter Zijlstra
2018-02-09  8:12                           ` [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f Peter Zijlstra
2018-02-13 11:30                         ` [tip:x86/pti] x86/debug: Use UD2 for WARN() tip-bot for Peter Zijlstra
2018-02-15  0:27                         ` tip-bot for Peter Zijlstra
2018-02-07 18:38   ` [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f Randy Dunlap
2018-02-07 19:01     ` Linus Torvalds
2018-02-07 19:06       ` Peter Zijlstra
2018-02-07 19:10         ` Peter Zijlstra [this message]

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