Message-ID: <CAKgNAkiQUXFZ82jDNqEPxpBmdkKOg03uQXg=iEuUNzg0rvgkZA@mail.gmail.com>
Date:	Sat, 10 Jan 2015 14:23:06 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	"Theodore Ts'o" <tytso@....edu>,
	linux-man <linux-man@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] getrandom.2: new manpage

Hi Heinrich,

Things seem to have stalled on the getrandom(2) man page (no response
to my mail below), and also in your conversation with Ted regarding
the 29 Nov patch proposal "urandom: handle signals immediately".
What's the current status?

Cheers,

Michael


On 11 November 2014 at 12:44, Michael Kerrisk (man-pages)
<mtk.manpages@...il.com> wrote:
> Hi Heinrich,
>
> On Fri, Oct 3, 2014 at 2:15 AM, Heinrich Schuchardt <xypron.glpk@....de> wrote:
>> Kernel 3.17 introduces a new system call getrandom(2).
>>
>> The man page in this patch is based on the commit message by
>> Theodore Ts'o <tytso@....edu> and suggestion by
>> Michael Kerrisk <mtk.manpages@...il.com>.
>
> No word from Ted so far...
>
> I've added LKML to CC, because I think it's worth getting wider review
> of the page at this point.
>
> I've done some further editing on the page, and pushed the current
> version in public branch:
> http://git.kernel.org/cgit/docs/man-pages/man-pages.git/log/?h=draft_getrandom
>
> Could you please review the current version, appended below. Note that
> there are a couple of FIXMEs there. Also, could you pay special
> attention to the changes in these commits, to make sure I have not
> injected any errors:
>
> pick 0ef180e getrandom.2: Add a sentence to clarify the default behavior...
> pick 62342ef getrandom.2: Reword GRND_NONBLOCK description
> pick 0c90d3d getrandom.2: Reword GRND_RANDOM description
>
> Cheers,
>
> Michael
>
> .\" Copyright (C) 2014, Theodore Ts'o <tytso@....edu>
> .\" Copyright (C) 2014, Heinrich Schuchardt <xypron.glpk@....de>
> .\"
> .\" %%%LICENSE_START(VERBATIM)
> .\" Permission is granted to make and distribute verbatim copies of this
> .\" manual provided the copyright notice and this permission notice are
> .\" preserved on all copies.
> .\"
> .\" Permission is granted to copy and distribute modified versions of
> .\" this manual under the conditions for verbatim copying, provided that
> .\" the entire resulting derived work is distributed under the terms of
> .\" a permission notice identical to this one.
> .\"
> .\" Since the Linux kernel and libraries are constantly changing, this
> .\" manual page may be incorrect or out-of-date.  The author(s) assume.
> .\" no responsibility for errors or omissions, or for damages resulting.
> .\" from the use of the information contained herein.  The author(s) may.
> .\" not have taken the same level of care in the production of this.
> .\" manual, which is licensed free of charge, as they might when working.
> .\" professionally.
> .\"
> .\" Formatted or processed versions of this manual, if unaccompanied by
> .\" the source, must acknowledge the copyright and authors of this work.
> .\" %%%LICENSE_END
>
> .TH GETRANDOM 2 2014-10-03 "Linux" "Linux Programmer's Manual"
> .SH NAME
> getrandom \- obtain a series of random bytes
> .SH SYNOPSIS
> .B #include <linux/random.h>
> .sp
> .BI "int getrandom(void *"buf ", size_t " buflen ", unsigned int " flags );
> .SH DESCRIPTION
> The system call
> .BR getrandom ()
> fills the buffer pointed to by
> .I buf
> with up to
> .I buflen
> random bytes.
> These can be used to seed user-space random number generators
> or for other cryptographic purposes.
> .PP
> .BR getrandom ()
> relies on entropy gathered from device drivers and other sources of
> environmental noise.
> Unnecessarily reading large quantities of data will have a negative impact
> on other users of the
> .I /dev/random
> and
> .I /dev/urandom
> devices.
> Therefore
> .BR getrandom ()
> should not be used for Monte Carlo simulations or other
> programs/algorithms which are doing probabilistic sampling.
>
> .\" FIXME is the following paragraph correct?
> By default,
> .BR getrandom ()
> draws entropy from the
> .IR /dev/urandom
> pool, and, if that pool has been initialized and
> .IR buflen
> is less than or equal to 256 (see NOTES, below),
> then the call never blocks when drawing from that pool
> and always returns the number of bytes requested in
> .IR buflen .
> This behavior can be changed via the
> .I flags
> argument.
>
> The
> .I flags
> argument is a bit mask that can contain zero or more of the following values
> ORed together:
> .TP
> .B GRND_RANDOM
> If this bit is set, then random bytes are drawn from the
> .I /dev/random
> pool instead of the
> .I /dev/urandom
> pool.
> The
> .I /dev/random
> pool is limited based on the entropy that can be obtained from environmental
> noise.
> If the number of available bytes in
> .I /dev/random
> is less than requested in
> .IR buflen ,
> the call returns just the available random bytes.
> If no random byte is available, the response will depend on the
> presence of
> .B GRND_NONBLOCK
> in the
> .I flags
> argument.
> .TP
> .B GRND_NONBLOCK
> By default, if there is no random byte available at all,
> .BR getrandom ()
> blocks until data is available.
> If the
> .B GRND_NONBLOCK
> flag is set, then
> .BR getrandom ()
> instead immediately returns -1 with
> .I errno
> set to
> .BR EAGAIN .
> .SH RETURN VALUE
> On success,
> .BR getrandom ()
> returns the number of bytes that were copied to the buffer
> .IR buf .
> This may be less than the number of bytes requested via
> .I buflen
> if insufficient entropy was present in the
> .IR /dev/random
> pool, or if the system call was interrupted by a signal.
> .PP
> On error, -1 is returned, and
> .I errno
> is set appropriately.
> .SH ERRORS
> .TP
> .B EINVAL
> An invalid flag was specified in
> .IR flags .
> .TP
> .B EFAULT
> The address referred to by
> .I buf
> is outside the accessible address space.
> .TP
> .B EAGAIN
> The requested entropy was not available, and
> .BR getrandom ()
> would have blocked if the
> .B GRND_NONBLOCK
> flag was not set.
> .TP
> .B EINTR
> While blocked waiting for entropy, the call was interrupted by a signal
> handler; see the description of how interrupted
> .BR read (2)
> calls on "slow" devices are handled with and without the
> .B SA_RESTART
> flag in the
> .BR signal (7)
> man page.
> .SH VERSIONS
> .BR getrandom ()
> was introduced in version 3.17 of the Linux kernel.
> .SH CONFORMING TO
> This system call is Linux-specific.
> .SH NOTES
> .SS Interruption by a signal handler
> .\" FIXME Here, I think there needs to be an opening paragraph that describes
> .\" the cases where getrandom() can block. This should cover the cases with
> .\" GRND_RANDOM and without GRND_RANDOM. Reading the existing page, I am
> .\" still not completely confident that I know what the cases are.
> The reaction of
> .BR getrandom ()
> in case of an interruption of a blocking call by a signal
> when reading from
> .I /dev/urandom
> .RB ( GRND_RANDOM
> is not set)
> depends on the initialization state of the entropy buffer
> and on the request size
> .IR buflen .
> If the entropy is not yet initialized or the request size is large
> .RI ( buflen "\ >\ 256),"
> .B EINTR
> will be returned.
> If the entropy pool has been initialized and the request size is small
> .RI ( buflen "\ <=\ 256),"
> .BR getrandom ()
> will not return
> .BR EINTR .
> Instead, it will return all of the bytes that have been requested.
> .PP
> When reading from
> .I /dev/random
> .RB ( GRND_RANDOM
> is set)
> these guarantees do
> .I not
> apply.
> .PP
> Calling
> .BR getrandom ()
> to read
> .I /dev/urandom
> for small values (<=\ 256) of
> .I buflen
> is the preferred mode of usage.
> .PP
> The special treatment of small values of
> .I buflen
> was designed for compatibility with
> OpenBSD's
> .BR getentropy ()
> system call.
> .PP
> The user of
> .BR getrandom ()
> .I must
> always check the return value,
> to determine whether either an error occurred
> or fewer bytes than requested were returned.
> In the case where
> .B GRND_RANDOM
> is not specified and
> .I buflen
> is less than or equal to 256,
> a return of fewer bytes than requested should never happen,
> but the careful user-space code should check for this anyway!
> .SS Choice of random device
> Unless you are doing long-term key generation (and perhaps not even
> then), you probably shouldn't be using
> .B GRND_RANDOM.
> The cryptographic algorithms used for
> .I /dev/urandom
> are quite conservative, and so should be sufficient for all purposes.
> The disadvantage of
> .B GRND_RANDOM
> is that it can block.
> Furthermore, dealing with partially fulfilled
> .BR getrandom ()
> requests increases code complexity.
> .SS Emulating OpenBSD's getentropy()
> The
> .BR getentropy ()
> system call in OpenBSD can be emulated using the following
> function:
>
> .in +4n
> .nf
> int
> getentropy(void *buf, size_t buflen)
> {
>     int ret;
>
>     if (buflen > 256)
>         goto failure;
>     ret = getrandom(buf, buflen, 0);
>     if (ret < 0)
>         return ret;
>     if (ret == buflen)
>         return 0;
> failure:
>     errno = EIO;
>     return -1;
> }
> .fi
> .in
> .SH SEE ALSO
> .BR random (4),
> .BR urandom (4)



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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