linux-kernel - Re: [PATCH v2 3/5] MIPS: Only change $28 to thread

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <alpine.DEB.2.00.1612150120400.6743@tp.orcam.me.uk>
Date:   Thu, 15 Dec 2016 11:51:54 +0000
From:   "Maciej W. Rozycki" <macro@...tec.com>
To:     Matt Redfearn <matt.redfearn@...tec.com>
CC:     Ralf Baechle <ralf@...ux-mips.org>, <linux-mips@...ux-mips.org>,
        "Jason A . Donenfeld" <Jason@...c4.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        <linux-kernel@...r.kernel.org>,
        James Hogan <james.hogan@...tec.com>,
        Paul Burton <paul.burton@...tec.com>
Subject: Re: [PATCH v2 3/5] MIPS: Only change $28 to thread_info if coming
 from user mode

On Tue, 13 Dec 2016, Matt Redfearn wrote:

> diff --git a/arch/mips/include/asm/stackframe.h b/arch/mips/include/asm/stackframe.h
> index eebf39549606..e13164daf9c2 100644
> --- a/arch/mips/include/asm/stackframe.h
> +++ b/arch/mips/include/asm/stackframe.h
> @@ -216,12 +216,20 @@
>  		LONG_S	$25, PT_R25(sp)
>  		LONG_S	$28, PT_R28(sp)
>  		LONG_S	$31, PT_R31(sp)
> +
> +		/* Set thread_info if we're coming from user mode */
> +		mfc0	k0, CP0_STATUS
> +		sll	k0, 3		/* extract cu0 bit */
> +		bltz	k0, 9f
> +		 nop
> +

 Have you actually verified the machine code generated as expected with 
such a change?  This supposedly delay-slot NOP can be dropped as this is 
`.set reorder' code, so GAS will have scheduled a NOP itself already, 
having found there is an SLL->BLTZ data dependency and consequently the 
two instructions cannot be swapped.  So now there'll be an extraneous NOP 
there, wasting code space and execution time.

  Maciej