linux-kernel - Re: Strange EFAULT on mips64el returned by syscall when another thread is forking

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Message-Id: <00d1b813-c55f-4365-8d81-d70258e10b16@app.fastmail.com>
Date: Thu, 25 Jan 2024 09:28:41 +0000
From: "Jiaxun Yang" <jiaxun.yang@...goat.com>
To: "Linus Torvalds" <torvalds@...ux-foundation.org>,
 "Xi Ruoyao" <xry111@...111.site>, tsbogend@...ha.franken.de
Cc: "Andreas Schwab" <schwab@...e.de>, "Ben Hutchings" <ben@...adent.org.uk>,
 "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
 linux-kernel@...r.kernel.org, libc-alpha@...rceware.org
Subject: Re: Strange EFAULT on mips64el returned by syscall when another thread is
 forking

在2024年1月24日一月 下午10:10，Linus Torvalds写道：
[...]
>
> Anyway, I'm pretty sure this is the bug, now some MIPS person just
> needs to fix the MIPS version of "instruction_pointer()" to do what
> "exception_epc()" already does.

Hi folks,

Kinda MIPS person here, I looked into the problem, and it's not that
easy to fix.

I inspected some existing usage of "instruction_pointer()", and some
of them do want exception return address (which is always CP0_EPC).
Others like this case they want the precise exception instruction
pointer ("exception_epc()" for MIPS).

I'm planning to make "instruction_pointer()" always point to exception
instructions, and implemented a new function called "return_pc()"or
whatever for "exception_epc()". Do you have a better name in mind?

Besides isa16 stuff do require kernel to read user space fault
instruction to determine delay slot size... I don't think it's always
possible when "instruction_pointer()" is called. MIPS16/microMIPS
is rarely used today though.

Thanks

>
>               Linus

-- 
- Jiaxun