linux-kernel - Re: [PATCH v2 0/3] Handle delay slot for extable lookup

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Date: Fri, 2 Feb 2024 21:31:22 +0000
From: Jiaxun Yang <jiaxun.yang@...goat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>,
 Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc: Oleg Nesterov <oleg@...hat.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Ben Hutchings <ben@...adent.org.uk>,
 linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-mips@...r.kernel.org, linux-mm@...ck.org,
 Xi Ruoyao <xry111@...111.site>
Subject: Re: [PATCH v2 0/3] Handle delay slot for extable lookup



在 2024/2/2 18:39, Linus Torvalds 写道:
> On Fri, 2 Feb 2024 at 04:30, Jiaxun Yang <jiaxun.yang@...goat.com> wrote:
>>        ptrace: Introduce exception_ip arch hook
>>        MIPS: Clear Cause.BD in instruction_pointer_set
>>        mm/memory: Use exception ip to search exception tables
> Just to clarify: does that second patch fix the problem that
> __isa_exception_epc() does a __get_user()?

No it only covers an obvious case when I was playing around exception_ip 
with kgdb.
There are still potential cases __isa_exception_epc may touch user space.

> Because that mm/memory.c use of "exception_ip()" most definitely
> cannot take a page fault.
>
> So if MIPS cannot do that whole exception IP thing without potentially
> touching user space, I do worry that we might just have to add a
>
>     #ifndef __MIPS__
>
> around this all.

It is possible to perform exception_ip() without touching user space by 
saving "BadInstr" in pt_regs
at exception entries. For newer hardware it's as simple as saving an 
extra CP0 register, on older hardware
we may have to read it from user space.

+ Thomas (MIPS maintainer), what's your opinion?

Thanks
>
> Possibly somehow limited to just the microMIPS/MIPS16e case in Kconfig instead?
>
>              Linus

-- 
---
Jiaxun Yang