linux-kernel - RE: [tip:x86/asm] x86/entry/64: Add two more instruction suffixes

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

Date:   Tue, 3 Jul 2018 08:46:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "jbeulich@...e.com" <jbeulich@...e.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "brgerst@...il.com" <brgerst@...il.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "dvlasenk@...hat.com" <dvlasenk@...hat.com>,
        "linux-tip-commits@...r.kernel.org" 
        <linux-tip-commits@...r.kernel.org>
Subject: RE: [tip:x86/asm] x86/entry/64: Add two more instruction suffixes

From: Jan Beulich
> Sent: 03 July 2018 09:36
...
> As said there, omitting suffixes from instructions in AT&T mode is bad
> practice when operand size cannot be determined by the assembler from
> register operands, and is likely going to be warned about by upstream
> gas in the future (mine does already).
...
> -	bt	$9, EFLAGS(%rsp)		/* interrupts off? */
> +	btl	$9, EFLAGS(%rsp)		/* interrupts off? */

Hmmm....
Does the operand size make any difference at all for the bit instructions?
I'm pretty sure that the cpus (386 onwards) have always done aligned 32bit
transfers (the docs never actually said aligned).
I can't remember whether 64bit mode allows immediates above 31.

So gas accepting 'btb $n,memory' is giving a false impression of
what actually happens.

	David