linux-kernel - Re: [PATCH, RFC] random: introduce getrandom(2) system call

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Message-ID: <20140717194812.GC24196@lenny.home.zabbo.net>
Date:	Thu, 17 Jul 2014 12:48:12 -0700
From:	Zach Brown <zab@...bo.net>
To:	Theodore Ts'o <tytso@....edu>
Cc:	linux-kernel@...r.kernel.org, linux-abi@...r.kernel.org,
	linux-crypto@...r.kernel.org, beck@...nbsd.org
Subject: Re: [PATCH, RFC] random: introduce getrandom(2) system call

> SYNOPSIS
> 	#include <linux/random.h>
> 
> 	int getrandom(void *buf, size_t buflen, unsigned int flags);

I certainly like the idea of getting entropy without having to worry
about fds.

> 	If the GRND_RANDOM flags bit is not set, then the /dev/raundom

(raundom typo)

> RETURN VALUE
>        On success, the number of bytes that was returned is returned.

The description talks about filling the buffer, maybe say 'the number of
bytes filled is returned'?  

> +DECLARE_COMPLETION(urandom_initialized);

static?

> +SYSCALL_DEFINE3(getrandom, char __user *, buf, size_t, count,
> +		unsigned int, flags)
> +{
> +	int r;
> +

Michael Kerrisk wants you to return -EINVAL on unknown flags :)

http://lwn.net/Articles/588444/

> +	if (count > 256)
> +		return -EINVAL;

I'd vote for not having the limit.  It seems easy enough to iterate over
the buffer.  We'd need to clamp the count to ssize_t, though.

> +	if (flags & GRND_RANDOM) {
> +		return _random_read(!(flags & GRND_BLOCK), buf, count);
> +	}

Do we want it to block by default and have the flag be _NONBLOCK?  Feels
more.. familiar.

> +	if (flags & GRND_BLOCK) {
> +		r = wait_for_completion_interruptible(&urandom_initialized);
> +		if (r)
> +			return r;

I can *never* remember the rules for -ERESTARTSYS.  The syscall callers
take care of this?

> +	return urandom_read(NULL, buf, count, NULL);

I wonder if we want to refactor the entry points a bit more instead of
directly calling the device read functions.  get_random_bytes() and
urandom_read() both have their own uninitialied use warning message and
tracing.  Does the syscall want its own little extraction function as
well?

- z
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