linux-kernel - Re: [RFC] metadata updates vs. fetches (was Re: [PATCH v4] fs: Fix data race in inode_set_ctime_to

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Message-ID: <20241124234111.GD3387508@ZenIV>
Date: Sun, 24 Nov 2024 23:41:11 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	"Darrick J. Wong" <djwong@...nel.org>,
	Hao-ran Zheng <zhenghaoran@...a.edu.cn>, brauner@...nel.org,
	jack@...e.cz, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, baijiaju1990@...il.com,
	21371365@...a.edu.cn
Subject: Re: [RFC] metadata updates vs. fetches (was Re: [PATCH v4] fs: Fix
 data race in inode_set_ctime_to_ts)

On Mon, Nov 25, 2024 at 12:19:44AM +0100, Mateusz Guzik wrote:

> + *
> + * DONTFIXME: no effort is put into ensuring a consistent snapshot of the
> + * metadata read below. For example a call racing against parallel setattr()
> + * can end up with a mixture of old and new attributes. This is not considered
> + * enough to warrant fixing.
>   */
>  void generic_fillattr(struct mnt_idmap *idmap, u32 request_mask,
>                       struct inode *inode, struct kstat *stat)
> 
> not an actual patch submission, any party is free to take the comment
> and tweak in whatever capacity without credit.
> 
> What I am after here is preventing more people from spotting the
> problem and thinking it is new.

getattr() is not the only reader for those - permission() is *much*
hotter; for uid/gid issues, if somebody can think of any scenario
where it's a real problem, permission() would be the main source
of headache.