linux-kernel - Re: [PATCH] eventfd: strictly check the count parameter of eventfd

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Message-ID: <20240208043354.GA85799@sol.localdomain>
Date: Wed, 7 Feb 2024 20:33:54 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: wenyang.linux@...mail.com
Cc: Christian Brauner <brauner@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Matthew Wilcox <willy@...radead.org>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] eventfd: strictly check the count parameter of
 eventfd_write to avoid inputting illegal strings

On Wed, Feb 07, 2024 at 12:35:18AM +0800, wenyang.linux@...mail.com wrote:
> By checking whether count is equal to sizeof(ucnt), such errors
> could be detected. It also follows the requirements of the manual.

Does it?  This is what the eventfd manual page says:

     A write(2) fails with the error EINVAL if the size of the supplied buffer
     is less than 8 bytes, or if an attempt is made to write the value
     0xffffffffffffffff.

So, *technically* it doesn't mention the behavior if the size is greater than 8
bytes.  But one might assume that such writes are accepted, since otherwise it
would have been mentioned that they're rejected, just like writes < 8 bytes.

If the validation is indeed going to be made more strict, the manual page will
need to be fixed alongside it.

- Eric