linux-kernel - RE: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of __put

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Date: Fri, 28 Jun 2024 15:36:17 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Arnd Bergmann' <arnd@...db.de>, Jisheng Zhang <jszhang@...nel.org>,
	Andreas Schwab <schwab@...e.de>
CC: Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt
	<palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>,
	"linux-riscv@...ts.infradead.org" <linux-riscv@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 2/4] riscv: uaccess: use input constraints for ptr of
 __put_user

From: Arnd Bergmann
> Sent: 26 June 2024 15:25
...
> If you just copy from the arm64 version that uses an
> "r"(address) constraint instead of the "m"(*address)
> version, it should be fine for any user space access.

Arm certainly has 'reg+offset' addressing and I'd have thought
the RISC-V would have it as well.

I'd guess that the compiler also knows when the offset is too big.

Probably noticeable when code is accessing structures in user memory.

OTOH I can't remember if "m" implies a memory clobber?
For user copies the memory clobber isn't needed and not having it
may well allow better instruction scheduling.

	David

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