Re: [PATCH] cdrom: Avoid barrier_nospec() in cdrom_ioctl_media_changed()

[Jens Axboe]

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