linux-kernel - Re: [PATCH 1/3] timerfd: convert to ->read

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Date: Wed, 3 Apr 2024 23:40:37 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Jens Axboe <axboe@...nel.dk>
Cc: linux-fsdevel@...r.kernel.org, brauner@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] timerfd: convert to ->read_iter()

On Wed, Apr 03, 2024 at 08:02:52AM -0600, Jens Axboe wrote:

> -		res = put_user(ticks, (u64 __user *) buf) ? -EFAULT: sizeof(ticks);
> +		res = copy_to_iter(&ticks, sizeof(ticks), to);

Umm...  That's not an equivalent transformation - different behaviour on
short copy; try to call it via read(fd, unmapped_buffer, 8) and see what
happens.

copy_to_iter() returns the amount copied; no data copied => return 0, not -EFAULT.

> +	ufd = get_unused_fd_flags(O_RDWR | (flags & TFD_SHARED_FCNTL_FLAGS));

You do realize that get_unused_fd_flags() ignores O_RDWR (or O_NDELAY), right?
Mixing those with O_CLOEXEC makes sense for anon_inode_getfd(), but here you
have separate calls of get_unused_fd_flags() and anon_inode_getfile(), so...