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From: Borislav Petkov <bp@alien8.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: kbuild test robot <fengguang.wu@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>,
	the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f
Date: Wed, 7 Feb 2018 19:35:43 +0100	[thread overview]
Message-ID: <20180207183543.GA8897@pd.tnic> (raw)
In-Reply-To: <CA+55aFxU7TArLUdcndeLnhfRKLO-Vu-1d0Q6VPb9NYt+-S-LNg@mail.gmail.com>

On Wed, Feb 07, 2018 at 10:13:35AM -0800, Linus Torvalds wrote:
> Adding more people for this funky warning from the kbuild robot.
> 
> Something is confused. UD0 is 0f ff, the bytes after that shouldn't
> matter. But I guess they can be interpreted as modrm bytes, and
> somebody started doing that.
> 
> That said, intel only _documents_ UD2 (0f 0b).

They documented UD0 and UD1 a year ago or so:

0F FF /r UD0¹  r32, r/m32 RM Valid Valid Raise invalid opcode exception
0F B9 /r UD1 r32, r/m32 RM Valid Valid Raise invalid opcode exception.

and the footnote says

"1. Some older processors decode the UD0 instruction without a ModR/M
byte. As a result, those processors would deliver an invalid- opcode
exception instead of a fault on instruction fetch when the instruction
with a ModR/M byte (and any implied bytes) would cross a page or segment
boundary."

So those two take a ModRM byte.

And we chose UD0 for WARN, see arch/x86/include/asm/bug.h for the
reasoning.

Except objdump can't handle that insn because it doesn't have it in its
insn tables. Thus it says:

  b3:   0f ff                   (bad)
  b5:   eb                      .byte 0xeb

> Maybe we should avoid using UD0/UD1 entirely.

Or that test should ignore UD0.

Or we should add UD0 only *decoding* support to binutils - not
generating.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

  reply	other threads:[~2018-02-07 18:36 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 [this message]
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

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