linux-kernel - Re: [PATCH] cdrom: Avoid barrier_nospec() in cdrom_ioctl_media

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Message-ID: <dd1712dd-1485-417a-81cc-482d7dd26a11@kernel.dk>
Date: Thu, 17 Oct 2024 18:33:24 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Josh Poimboeuf <jpoimboe@...nel.org>,
 Phillip Potter <phil@...lpotter.co.uk>
Cc: linux-kernel@...r.kernel.org, Jordy Zomer <jordyzomer@...gle.com>
Subject: Re: [PATCH] cdrom: Avoid barrier_nospec() in
 cdrom_ioctl_media_changed()

On 10/17/24 4:09 PM, Josh Poimboeuf wrote:
> The barrier_nospec() after the array bounds check is overkill and
> painfully slow for arches which implement it.
> 
> Furthermore, most arches don't implement it, so they remain exposed to
> Spectre v1 (which can affect pretty much any CPU with branch
> prediction).
> 
> Instead, clamp the user pointer to a valid range so it's guaranteed to
> be a valid array index even when the bounds check mispredicts.

It's a cdrom, and media change detection to be more specific. I really
don't think anyone would care about performance here, it's not even
a hot path for a cdrom driver. That said, I don't disagree with
the change, just don't think it'll make one iota of difference
in the real world.

-- 
Jens Axboe