linux-kernel - Re: [PATCH 3/4] ublk: use READ_ONCE() to read struct ublksrv_ctrl

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Message-ID: <aXzUoRgvbE0izVVP@fedora>
Date: Fri, 30 Jan 2026 23:56:17 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Caleb Sander Mateos <csander@...estorage.com>
Cc: Jens Axboe <axboe@...nel.dk>,
	Govindarajulu Varadarajan <govind.varadar@...il.com>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] ublk: use READ_ONCE() to read struct ublksrv_ctrl_cmd

On Thu, Jan 29, 2026 at 03:46:16PM -0700, Caleb Sander Mateos wrote:
> struct ublksrv_ctrl_cmd is part of the io_uring_sqe, which may lie in
> userspace-mapped memory. It's racy to access its fields with normal
> loads, as userspace may write to them concurrently. Use READ_ONCE() for
> all the ublksrv_ctrl_cmd field accesses to avoid the race.
> 
> Fixes: 87213b0d847c ("ublk: allow non-blocking ctrl cmds in IO_URING_F_NONBLOCK issue")
> Signed-off-by: Caleb Sander Mateos <csander@...estorage.com>
> ---
>  drivers/block/ublk_drv.c | 77 +++++++++++++++++++---------------------
>  1 file changed, 37 insertions(+), 40 deletions(-)

I'd suggest to define local `struct ublksrv_ctrl_cmd` variable in
ublk_ctrl_uring_cmd() for avoiding READ_ONCE:

- it is slow control cod path

- READ_ONCE is supposed to be used for fast path and be carefully

- it is sync command

Defining one local data can be simpler to avoid the issue, and more
reliable.


Thanks,
Ming