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Re: [PATCH v2] x86: Stop relying on magic jmp behavior for early_idt_handlers
- From: Andy Lutomirski <luto at amacapital dot net>
- To: Ingo Molnar <mingo at kernel dot org>
- Cc: Andy Lutomirski <luto at kernel dot org>, X86 ML <x86 at kernel dot org>, "H. Peter Anvin" <hpa at zytor dot com>, "H.J. Lu" <hjl dot tools at gmail dot com>, Borislav Petkov <bp at alien8 dot de>, Jan Beulich <JBeulich at suse dot com>, Binutils <binutils at sourceware dot org>, "linux-kernel at vger dot kernel dot org" <linux-kernel at vger dot kernel dot org>
- Date: Thu, 21 May 2015 23:51:50 -0700
- Subject: Re: [PATCH v2] x86: Stop relying on magic jmp behavior for early_idt_handlers
- Authentication-results: sourceware.org; auth=none
- References: <9b741597a52258e829bae247216da656d452395a dot 1432257964 dot git dot luto at kernel dot org> <20150522062224 dot GA4736 at gmail dot com>
On Thu, May 21, 2015 at 11:22 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Andy Lutomirski <luto@kernel.org> wrote:
>
>> --- a/arch/x86/include/asm/segment.h
>> +++ b/arch/x86/include/asm/segment.h
>> @@ -231,9 +231,17 @@
>> #define TLS_SIZE (GDT_ENTRY_TLS_ENTRIES* 8)
>>
>> #ifdef __KERNEL__
>> +
>> +/*
>> + * early_idt_handlers is an array of entry points. For simplicity, it's
>> + * a real array. We allocate nine bytes for each entry: two one-byte
>> + * push instructions and a five-byte jump in the worst case.
>> + */
>> +#define EARLY_IDT_HANDLER_STRIDE 9
>
> So how come that two plus five equals nine? ;-)
A "one-byte push" would be pretty impressive :)
>
> It's two two-bype pushes (sometimes a 2-byte NOP), plus a 5-byte jump,
> in the worst case.
>
> I'd also mention that it's an array of 32 small trampolines that set
> up parameters and jump to a common entry point.
Will do.
>
>> +/* Build the early_idt_handlers array */
>> ENTRY(early_idt_handlers)
>
> Please rename the function accordingly: early_idt_handlers_array, to
> make clear this is never jumped to directly, only through the IDT.
Makes sense.
>
>> # 36(%esp) %eflags
>> # 32(%esp) %cs
>> @@ -531,19 +532,18 @@ ENTRY(early_idt_handlers)
>> # 24(%rsp) error code
>> i = 0
>> .rept NUM_EXCEPTION_VECTORS
>> - .if (EXCEPTION_ERRCODE_MASK >> i) & 1
>> - ASM_NOP2
>> - .else
>> + .fill early_idt_handlers + i * EARLY_IDT_HANDLER_STRIDE - ., 1, 0xcc
>> + .ifeq (EXCEPTION_ERRCODE_MASK >> i) & 1
>> pushl $0 # Dummy error code, to make stack frame uniform
>> .endif
>> pushl $i # 20(%esp) Vector number
>> jmp early_idt_handler
>> i = i + 1
>> .endr
>> + .fill early_idt_handlers + i * EARLY_IDT_HANDLER_STRIDE - ., 1, 0xcc
>> ENDPROC(early_idt_handlers)
>>
>> - /* This is global to keep gas from relaxing the jumps */
>> -ENTRY(early_idt_handler)
>> +early_idt_handler:
>
> Please rename this as well to make it clearer what's happening,
> something like:
>
> early_idt_handler_common:
>
> would work for me.
>
> Ditto for the 64-bit side.
Sounds good. I'll do all this tomorrow and resend.
--Andy