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

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Message-Id: <3ac65601-7102-4ad1-ad89-157f7d9cef87@app.fastmail.com>
Date: Mon, 25 Mar 2024 21:40:41 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Samuel Holland" <samuel.holland@...ive.com>,
 "Palmer Dabbelt" <palmer@...belt.com>, linux-riscv@...ts.infradead.org
Cc: "Albert Ou" <aou@...s.berkeley.edu>,
 "Alexandre Ghiti" <alexghiti@...osinc.com>,
 "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
Subject: Re: [PATCH] riscv: Define TASK_SIZE_MAX for __access_ok()

On Wed, Mar 13, 2024, at 18:59, Samuel Holland wrote:
> TASK_SIZE_MAX should be set to the largest userspace address under any
> runtime configuration. This optimizes the check in __access_ok(), which
> no longer needs to compute the current value of TASK_SIZE. It is still
> safe because addresses between TASK_SIZE and TASK_SIZE_MAX are invalid
> at the hardware level.
>
> This removes about half of the references to pgtable_l[45]_enabled.
>
> Signed-off-by: Samuel Holland <samuel.holland@...ive.com>

Reviewed-by: Arnd Bergmann <arnd@...db.de>

>  #ifdef CONFIG_64BIT
>  #define TASK_SIZE_64	(PGDIR_SIZE * PTRS_PER_PGD / 2)
>  #define TASK_SIZE_MIN	(PGDIR_SIZE_L3 * PTRS_PER_PGD / 2)
> +#define TASK_SIZE_MAX	(PGDIR_SIZE_L5 * PTRS_PER_PGD / 2)

I see that TASK_SIZE_MIN is unused since 085e2ff9aeb0 ("efi:
libstub: Drop randomization of runtime memory map") and could
be dropped now, but it doesn't really hurt either.

     Arnd