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Re: [PATCH v2] x86: Stop relying on magic jmp behavior for early_idt_handlers
- From: Ingo Molnar <mingo at kernel dot org>
- To: Andy Lutomirski <luto at kernel dot org>
- Cc: 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: Fri, 22 May 2015 08:22:24 +0200
- 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>
* 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? ;-)
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.
> +/* 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.
> # 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.
Thanks,
Ingo