linux-kernel - Re: [PATCH] riscv: Define TASK_SIZE_MAX for __access

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Date: Mon, 25 Mar 2024 21:38:20 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Mark Rutland" <mark.rutland@....com>
Cc: "Alexandre Ghiti" <alex@...ti.fr>,
 "David Laight" <David.Laight@...lab.com>,
 "Samuel Holland" <samuel.holland@...ive.com>,
 "Alexandre Ghiti" <alexghiti@...osinc.com>,
 "Palmer Dabbelt" <palmer@...belt.com>,
 "linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
 "Albert Ou" <aou@...s.berkeley.edu>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Charlie Jenkins" <charlie@...osinc.com>, guoren <guoren@...nel.org>,
 "Jisheng Zhang" <jszhang@...nel.org>,
 "Kemeng Shi" <shikemeng@...weicloud.com>,
 "Matthew Wilcox" <willy@...radead.org>, "Mike Rapoport" <rppt@...nel.org>,
 "Paul Walmsley" <paul.walmsley@...ive.com>,
 "Xiao W Wang" <xiao.w.wang@...el.com>, "Yangyu Chen" <cyy@...self.name>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] riscv: Define TASK_SIZE_MAX for __access_ok()

On Mon, Mar 25, 2024, at 19:30, Mark Rutland wrote:
> On Mon, Mar 25, 2024 at 07:02:13PM +0100, Arnd Bergmann wrote:
>> On Mon, Mar 25, 2024, at 17:39, Mark Rutland wrote:
>
>> If an architecture ignores all the top bits of a virtual address,
>> the largest TASK_SIZE would be higher than the smallest (positive,
>> unsigned) PAGE_OFFSET, so you need TASK_SIZE_MAX to be dynamic.
>
> Agreed, but do we even support such architectures within Linux?

Apparently not.

On 32-bit architectures, you often have TASK_SIZE==PAGE_OFFSET,
but not on 64-bit -- either the top few bits in PAGE_OFFSET are
always ones, or the user and kernel page tables are completely
separate.

>> It doesn't look like this is the case on riscv, but I'm not sure
>> about this part.
>
> It looks like riscv is in the same bucket as arm64 and x86 per:
>
>   https://www.kernel.org/doc/html/next/riscv/vm-layout.html
>
> ... which says:
>
> | The RISC-V privileged architecture document states that the 64bit addresses
> | "must have bits 63-48 all equal to bit 47, or else a page-fault exception
> | will occur.": that splits the virtual address space into 2 halves separated
> | by a very big hole, the lower half is where the userspace resides, the upper
> | half is where the RISC-V Linux Kernel resides.

Rihgt. I had even looked in that directory but somehow missed
the vm-layout.rst file.

     Arnd