systemstack stop traceback

wangjin...@gmail.com

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Jul 30, 2019, 11:58:54 PM7/30/19

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I read go runtime code，but I'm poor in assembly language,

i can't understand the following code

// switch to g0

MOVQ DX, g(CX)

MOVQ (g_sched+gobuf_sp)(DX), BX

// make it look like mstart called systemstack on g0, to stop traceback

SUBQ $8, BX

MOVQ $runtime·mstart(SB), DX

MOVQ DX, 0(BX)

MOVQ BX, SP

what does copy runtime·mstart to stack means,could someone help me

Ian Lance Taylor

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Jul 31, 2019, 12:08:51 AM7/31/19

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It does exactly what the comment says: when doing a stack traceback,
it makes it appear that mstart called systemstack. When a function is
called, the return address is placed on the stack. In this case the
assembly code is deliberately putting mstart on the stack, so that
that is what a traceback will see.

Ian

jin wang

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Jul 31, 2019, 4:45:01 AM7/31/19

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在 2019年7月31日星期三 UTC+8下午12:08:51，Ian Lance Taylor写道：

jin wang

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Jul 31, 2019, 7:49:39 AM7/31/19

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so mstart just put on stack for call traceback , but actually will not return at mstart?

在 2019年7月31日星期三 UTC+8下午12:08:51，Ian Lance Taylor写道：

On Tue, Jul 30, 2019 at 8:58 PM <wangjin...@gmail.com> wrote:

Ian Lance Taylor

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Jul 31, 2019, 9:49:04 AM7/31/19

to jin wang, golang-nuts

On Wed, Jul 31, 2019 at 4:49 AM jin wang <wangjin...@gmail.com> wrote:
>
> so mstart just put on stack for call traceback , but actually will not return at mstart?

Yes. As you can see, the systemstack function changes the stack
pointer back to its original value before it returns to its caller.
It does not return on the stack where mstart is stored.

Ian

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jin wang

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Jul 31, 2019, 10:06:20 PM7/31/19

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Thank you very much.

在 2019年7月31日星期三 UTC+8下午9:49:04，Ian Lance Taylor写道：

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