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From: Andy Lutomirski <luto@amacapital.net>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: X86 ML <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Borislav Petkov <bp@alien8.de>, Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK_PADDING macro
Date: Tue, 10 Mar 2015 12:47:53 -0700	[thread overview]
Message-ID: <CALCETrW5uiwJB0ox7jVcW+89e1yOHCnNQTR-AxgD-ed=D5EnXQ@mail.gmail.com> (raw)
In-Reply-To: <54FF4459.3010306@redhat.com>

On Tue, Mar 10, 2015 at 12:22 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> On 03/10/2015 07:05 PM, Andy Lutomirski wrote:
>> x86_32, unlike x86_64, pads the top of the kernel stack.  Document
>> this padding and give it a name.
>>
>> This should make no change whatsoever to the compiled kernel image.
>> It also doesn't fix any of the current bugs in this area.
>>
>> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
>> ---
>>  arch/x86/include/asm/processor.h   |  3 ++-
>>  arch/x86/include/asm/thread_info.h | 30 ++++++++++++++++++++++++++++++
>>  2 files changed, 32 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>> index 48a61c1c626e..88d9aa745898 100644
>> --- a/arch/x86/include/asm/processor.h
>> +++ b/arch/x86/include/asm/processor.h
>> @@ -849,7 +849,8 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
>>  #define task_pt_regs(task)                                             \
>>  ({                                                                     \
>>         struct pt_regs *__regs__;                                       \
>> -       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task))-8); \
>> +       __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task)) - \
>> +                                  TOP_OF_KERNEL_STACK_PADDING);     \
>>         __regs__ - 1;                                                   \
>>  })
>
> The "magic 8" also hides here:
>
>         /*
>          * Push current_thread_info()->sysenter_return to the stack.
>          * A tiny bit of offset fixup is necessary - 4*4 means the 4 words
>          * pushed above; +8 corresponds to copy_thread's esp0 setting.
>          */
>         pushl_cfi ((TI_sysenter_return)-THREAD_SIZE+8+4*4)(%esp)
>                                                    ^^^

I'll just put TOP_OF_KERNEL_STACK_PADDING there, too.

--Andy

>         CFI_REL_OFFSET eip, 0
>
>
> It may make sense to mention TOP_OF_KERNEL_STACK_PADDING here
> (there are no useful comments in copy_thread() anymore):
>
>         movl PT_EFLAGS(%esp), %eax      # mix EFLAGS, SS and CS
>         # Warning: PT_OLDSS(%esp) contains the wrong/random values if we
>         # are returning to the kernel.
>         # See comments in process.c:copy_thread() for details.
> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> maybe s/comments in process.c:copy_thread()/TOP_OF_KERNEL_STACK_PADDING/ ?
>         movb PT_OLDSS(%esp), %ah
>         movb PT_CS(%esp), %al
>         andl $(X86_EFLAGS_VM | (SEGMENT_TI_MASK << 8) | SEGMENT_RPL_MASK), %eax
>         cmpl $((SEGMENT_LDT << 8) | USER_RPL), %eax
>         CFI_REMEMBER_STATE
>



-- 
Andy Lutomirski
AMA Capital Management, LLC

  reply	other threads:[~2015-03-10 19:48 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-10 18:05 [PATCH 0/3] sp0, ss1, and sp1 docs and minor fixes Andy Lutomirski
2015-03-10 18:05 ` [PATCH 1/3] x86: Create and use a TOP_OF_KERNEL_STACK_PADDING macro Andy Lutomirski
2015-03-10 19:22   ` Denys Vlasenko
2015-03-10 19:47     ` Andy Lutomirski [this message]
2015-03-13 14:08     ` Denys Vlasenko
2015-03-16  8:56   ` Ingo Molnar
2015-03-16 12:08   ` [tip:x86/asm] x86/asm/entry: Create and use a ' TOP_OF_KERNEL_STACK_PADDING' macro tip-bot for Andy Lutomirski
2015-03-17  8:45   ` tip-bot for Andy Lutomirski
2015-03-10 18:05 ` [PATCH 2/3] x86: Unify and fix init sp0 Andy Lutomirski
2015-03-11 11:21   ` Borislav Petkov
2015-03-16 12:09   ` [tip:x86/asm] x86/asm/entry: Unify and fix initial thread_struct: :sp0 values tip-bot for Andy Lutomirski
2015-03-17  8:45   ` tip-bot for Andy Lutomirski
2015-03-10 18:06 ` [PATCH 3/3] x86_32: Document our abuse of ss1 and sp1 Andy Lutomirski
2015-03-10 19:13   ` Denys Vlasenko
2015-03-10 20:06     ` Andy Lutomirski
2015-03-10 20:52       ` Denys Vlasenko
2015-03-16 12:09   ` [tip:x86/asm] x86/asm/entry/32: Document our abuse of x86_hw_tss: :ss1 and x86_hw_tss::sp1 tip-bot for Andy Lutomirski
2015-03-16 15:36     ` Andy Lutomirski
2015-03-17  8:45   ` tip-bot for Andy Lutomirski

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