linux-kernel - Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions

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Date:	Tue, 31 Mar 2015 16:09:47 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Brian Gerst <brgerst@...il.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/9] x86/asm/entry/32: tidy up some instructions

On Tue, Mar 31, 2015 at 3:21 PM, Brian Gerst <brgerst@...il.com> wrote:
>>
>> @@ -708,7 +708,7 @@ END(sysenter_badsys)
>>  #ifdef CONFIG_X86_ESPFIX32
>>         movl %ss, %eax
>>         /* see if on espfix stack */
>> -       cmpw $__ESPFIX_SS, %ax
>> +       cmpl $__ESPFIX_SS, %eax
>>         jne 27f
>>         movl $__KERNEL_DS, %eax
>>         movl %eax, %ds
>
> This is incorrect.  32-bit reads from a segment register are not
> zero-extended.  The upper 16 bits are implementation-defined.  Most
> processors will clear them but it's not guaranteed.

Indeed. Brian is right. That cmpw needs to stay as a 16-bit compare.

                      Linus
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