linux-kernel - Re: [PATCH 2/2 v2] x86: make 32-bit "emergency stack" better documented

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Date:	Sat, 14 Mar 2015 13:24:25 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Pavel Machek <pavel@....cz>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>, 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 2/2 v2] x86: make 32-bit "emergency stack" better documented

On Sat, Mar 14, 2015 at 12:00 PM, Pavel Machek <pavel@....cz> wrote:
> Hi!
>
>>  arch/x86/kernel/asm-offsets_32.c | 2 +-
>>  arch/x86/kernel/cpu/common.c     | 3 ++-
>>  3 files changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>> index 48a61c1..9e65cf8 100644
>> --- a/arch/x86/include/asm/processor.h
>> +++ b/arch/x86/include/asm/processor.h
>> @@ -276,9 +276,9 @@ struct tss_struct {
>>       unsigned long           io_bitmap[IO_BITMAP_LONGS + 1];
>>
>>       /*
>> -      * .. and then another 0x100 bytes for the emergency kernel stack:
>> +      * and then space for temporary SYSENTER stack:
>>        */
>> -     unsigned long           stack[64];
>> +     unsigned long           SYSENTER_stack[64];
>
> Would it make sense to make it unsigned char ...stack[256], like the comment said?
>
> Or is it supposed to be different size on 64bit, and comment was wrong?

It isn't used on 64-bit.  MSR_IA32_SYSENTER_ESP is set to 0, and
tss->sp0 is read from the percpu segment instead of relative to the
stack pointer.

--
Brian Gerst
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