Hi Heinrich, On Fri, Oct 3, 2014 at 2:15 AM, Heinrich Schuchardt <xypron.glpk@xxxxxx> 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@xxxxxxx> and suggestion by > Michael Kerrisk <mtk.manpages@xxxxxxxxx>. 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@xxxxxxx> .\" Copyright (C) 2014, Heinrich Schuchardt <xypron.glpk@xxxxxx> .\" .\" %%%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 .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date.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) -- To unsubscribe from this list: send the line "unsubscribe linux-man" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
