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

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Date: Wed, 3 Apr 2024 23:57:47 +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 3/3] signalfd: convert to ->read_iter()

On Wed, Apr 03, 2024 at 08:02:54AM -0600, Jens Axboe wrote:
> Rather than use the older style ->read() hook, use ->read_iter() so that
> signalfd can support both O_NONBLOCK and IOCB_NOWAIT for non-blocking
> read attempts.
> 
> Split the fd setup into two parts, so that signalfd can mark the file
> mode with FMODE_NOWAIT before installing it into the process table.

Same issue with copy_to_iter() calling conventions; what's more, userland
really does not expect partial copies here, so it might be worth adding

static inline
bool copy_to_iter_full(void *addr, size_t bytes, struct iov_iter *i)
{
        size_t copied = copy_to_iter(addr, bytes, i);
	if (likely(copied == bytes))
		return true;
	iov_iter_revert(i, copied);
	return false;
}

to include/linux/uio.h for the sake of those suckers.  Then
they could go for
        return copy_to_iter_full(&new, sizeof(new), to) ? sizeof(new) : -EFAULT;
and similar in other two.

NOTE: the userland ABI is somewhat sucky here - if the buffer goes
unmapped (or r/o) at the offset that is *not* a multiple of
sizeof(struct signalfd_siginfo), you get an event quietly lost.
Not sure what can be done with that - it is a user-visible ABI.