Re: [PATCH v2 2/2] riscv: Define TASK_SIZE_MAX for __access_ok()

From: "Arnd Bergmann" <arnd@arndb.de>
To: "Samuel Holland" <samuel.holland@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	linux-riscv@lists.infradead.org
Cc: "Mark Rutland" <mark.rutland@arm.com>,
	"David Laight" <David.Laight@ACULAB.COM>,
	"Alexandre Ghiti" <alexghiti@rivosinc.com>
Subject: Re: [PATCH v2 2/2] riscv: Define TASK_SIZE_MAX for __access_ok()
Date: Wed, 27 Mar 2024 17:24:06 +0100	[thread overview]
Message-ID: <4d2fa4cd-4e21-4504-b29b-cfabe90693aa@app.fastmail.com> (raw)
In-Reply-To: <20240327143858.711792-3-samuel.holland@sifive.com>

On Wed, Mar 27, 2024, at 15:38, Samuel Holland wrote:
> TASK_SIZE_MAX should be set to a constant value, at least the largest
> valid userspace address under any runtime configuration. This optimizes
> the check in __access_ok(), which no longer needs to compute the runtime
> value of TASK_SIZE. The check does not need to be exact, as long as it
> accepts all valid userspace addresses and rejects all valid kernel
> addresses; well-behaved programs will never fail the access_ok() check.
>
> For RISC-V, which requires all virtual addresses to be sign extended,
> the optimal choice is LONG_MAX because it simplifies the limit
> comparison to a sign bit test.
>
> This removes about half of the references to pgtable_l[45]_enabled.
>
> Signed-off-by: Samuel Holland <samuel.holland@sifive.com>

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

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