linux-kernel - Re: [PATCH] signalfd: retrieve multiple signals with one read() call

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Date:	Sun, 20 May 2007 21:02:10 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Davi Arnaut <davi@...ent.com.br>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] signalfd: retrieve multiple signals with one read()
 call

On Sat, 19 May 2007 21:07:11 -0300 Davi Arnaut <davi@...ent.com.br> wrote:

> Hi,
> 
> Gathering signals in bulk enables server applications to drain a signal
> queue (almost full of realtime signals) more efficiently by reducing the
> syscall and file look-up overhead.
> 
> Very similar to the sigtimedwait4() call described by Niels Provos,
> Chuck Lever, and Stephen Tweedie in a paper entitled "Analyzing the
> Overload Behavior of a Simple Web Server". The paper lists more details
> and advantages.
> 

static ssize_t signalfd_read(struct file *file, char __user *buf, size_t count,
			     loff_t *ppos)
{
	struct signalfd_ctx *ctx = file->private_data;
	struct signalfd_siginfo __user *siginfo;
	int nonblock = file->f_flags & O_NONBLOCK;
	ssize_t ret, total = 0;
	siginfo_t info;

	count /= sizeof(struct signalfd_siginfo);
	if (!count)
		return -EINVAL;

	siginfo = (struct signalfd_siginfo __user *) buf;

	do {
		ret = signalfd_dequeue(ctx, &info, nonblock);
		if (unlikely(ret <= 0))
			break;
		ret = signalfd_copyinfo(siginfo, &info);
		if (ret < 0)
			break;
		siginfo++;
		total += ret;
		nonblock = 1;
	} while (--count);

	return total ? total : ret;
}

If 'count' is not a multiple of sizeof(struct signalfd_siginfo)), the read()
will return the next smallest multiple of `count'.

That is, unless `count' happens to be less than 1*sizeof(struct
signalfd_siginfo)), in which case we return -EINVAL.

This seems inconsistent.


Also, I'm desperately hunting for the place where we zero out that local
siginfo_t, and I ain't finding it.  Someone please convince me that we're
not leaking bits of kernel memory out to userspace in that thing.


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