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From: Vincent Chen <vincent.chen@sifive.com>
To: Romain Dolbeau <romain.dolbeau@european-processor-initiative.eu>
Cc: Carlos Eduardo de Paula <me@carlosedp.com>,
	Alex Ghiti <alex@ghiti.fr>,
	David Abdurachmanov <david.abdurachmanov@gmail.com>,
	Anup Patel <anup@brainfault.org>, Paul Walmsley <paul@pwsan.com>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	Aurelien Jarno <aurelien@aurel32.net>
Subject: Re: Error on loading some network Kernel modules
Date: Mon, 3 Feb 2020 18:04:11 +0800	[thread overview]
Message-ID: <CABvJ_xiWDZfO6fOd4Roiy-yaUVFKrGEcBDBSWsvj2TKGGMjy0g@mail.gmail.com> (raw)
In-Reply-To: <33c9a276-6212-115c-c7c9-e62244f13c76@european-processor-initiative.eu>

On Mon, Feb 3, 2020 at 12:27 AM Romain Dolbeau
<romain.dolbeau@european-processor-initiative.eu> wrote:
>
> On 2020-02-01 14:59, Alex Ghiti wrote:
> > Why restrict to 128M whereas we have 2GB offset available to the end of
> > the kernel ?
>
> Isn't that 2 GiB offset to whatever the module requires in the kernel,
> rather than to the end of the kernel space?
>
> Is there some guarantee that symbols accessible by modules are at the
> end of the kernel? If so, wouldn't the maximum offset for this patch
> still be (2 GiB - <total size of accessible symbols>)?
>
> Cordially,
>
> --
> Romain Dolbeau

It took me some time to find the root cause of this problem, please
allow me to share some observations before the discussion.
The root cause of this issue is that the percpu data is declared as a
static variable. The "static" attribute will make the compiler think
that this symbol is close to the .text section at runtime. Hence, the
compiler uses "auipc" to access this percpu data instead of using GOT.
In this case,  the access range is limited to + -2G. However, in
practice, these percpu data are placed at a pre-allocated region
created by the memblock_allocate() function. In other words, the
distance between the pre-allocated region (>PAGE_OFFSET ) and the
.text section of the kernel module (in VMALLOC region) is much larger
than 2G.
I agree that the original patch,
https://github.com/bjoto/linux/commit/8a56d1c8e8e91c1bc3893946d52b9217c96e1589,
can solve most cases. However, I do not think the patch still works if
the kernel module region is determined by _end or <total size of
accessible symbols>. The reason is that the pre-allocated region for
module percpu data comes from the memblock function at runtime. Hence,
we cannot know the actual address of this region at compile-time, and
this issue probably may occur again in this case.

By the way, I think maybe we can refer to the implementation of MIPS.
1. For general cases, we can use this patch to solve this issue.
2. For a large kernel image (>2G) or enabling the KASLR feature, we
may need a new code mode to deal with this issue.


  reply	other threads:[~2020-02-03 10:04 UTC|newest]

Thread overview: 30+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-28 19:22 Error on loading some network Kernel modules Carlos Eduardo de Paula
2020-01-30  3:00 ` Paul Walmsley
2020-01-30 16:20   ` David Abdurachmanov
2020-01-31 14:12     ` Carlos Eduardo de Paula
2020-01-31 20:11     ` Aurelien Jarno
2020-02-01  7:52       ` Anup Patel
2020-02-01 13:59         ` Alex Ghiti
2020-02-02 14:10           ` Anup Patel
2020-02-02 15:21             ` Troy Benjegerdes
2020-02-02 16:27           ` Romain Dolbeau
2020-02-03 10:04             ` Vincent Chen [this message]
2020-02-04  3:55               ` Zong Li
2020-02-04  6:50                 ` Alex Ghiti
2020-02-04  7:19                   ` Zong Li
2020-02-04  9:32                     ` Alexandre Ghiti
2020-02-04 10:46                       ` Vincent Chen
2020-02-04 11:30                         ` Anup Patel
2020-02-04 14:03                           ` Vincent Chen
2020-02-04 19:10                             ` Alex Ghiti
     [not found]                             ` <a55f265e-71b2-5ebb-b079-6345007a442e@ghiti.fr>
2020-02-05  3:22                               ` Vincent Chen
2020-02-05  4:24                                 ` Anup Patel
2020-02-05 10:37                                   ` Vincent Chen
2020-02-07 14:39                                     ` Vincent Chen
2020-02-07 14:51                                       ` Vincent Chen
2020-02-10  6:37                                         ` Alex Ghiti
2020-02-10  9:53                                           ` Vincent Chen
2020-02-19  6:46                                             ` Alex Ghiti
2020-02-19  7:30                                               ` Vincent Chen
2020-02-04 17:48                           ` Romain Dolbeau
2020-02-03 20:57             ` Alex Ghiti

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