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

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Message-Id: <eeccbc9f-7544-42c9-964f-2b4c924c2b2f@app.fastmail.com>
Date: Mon, 25 Mar 2024 19:02:13 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Mark Rutland" <mark.rutland@....com>, "Alexandre Ghiti" <alex@...ti.fr>
Cc: "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 17:39, Mark Rutland wrote:

> Using a compile-time constant TASK_SIZE_MAX allows the compiler to generate
> much better code for access_ok(), and on arm64 we use a compile-time constant
> even when our page table depth can change at runtime (and when native/compat
> task sizes differ). The only abosolute boundary that needs to be maintained is
> that access_ok() fails for kernel addresses.

As I understand, this works on arm64 and x86 because the kernel
mapping starts on negative 64-bit addresses, so the highest user
address (TASK_SIZE = 0x000fffffffffffff) is still smaller than the
lowest kernel address (PAGE_OFFSET = 0xfff0000000000000).

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.
It doesn't look like this is the case on riscv, but I'm not sure
about this part.

      Arnd