libc: Reintroduce getentropy.

This was introduced two years ago when the getrandom/getentropy API
question was still open, and removed because the discussion was
ongoing. Now getentropy is more wid

libc: Reintroduce getentropy.

This was introduced two years ago when the getrandom/getentropy API
question was still open, and removed because the discussion was
ongoing. Now getentropy is more widely adopted and soon to be in
POSIX. So reintroduce the symbol into libc since we'll be keeping it
anyway. Discussion of details of the semantics, as interpreted by
NetBSD, is ongoing, but the symbol needs to get in before the
netbsd-10 branch. The draft POSIX text is
(https://www.opengroup.org/austin/docs/austin_1110.pdf):

SYNOPSIS
#include <unistd.h>

int getentropy(void *buffer, size_t length);

DESCRIPTION
The getentropy() function shall write length bytes of data
starting at the location pointed to by buffer. The output
shall be unpredictable high quality random data, generated by
a cryptographically secure pseudo-random number
generator. The maximum permitted value for the length
argument is given by the {GETENTROPY_MAX} symbolic constant
defined in <limits.h>.

RETURN VALUES
Upon successful completion, getentropy() shall return 0;
otherwise, -1 shall be retunred and errno set to indicate the
error.

ERRORS
The getentropy() function shall fail if:

[EINVAL] The value of length is greater than
{GETENTROPY_MAX}.

The getentropy() function may fail if:

[ENOSYS] The system does not provide the necessary
source of entropy.

RATIONALE
The getentropy() function is not a cancellation point.

Minor changes from the previous introduction of getentropy into libc:

- Return EINVAL, not EIO, on buflen > 256.
- Define GETENTROPY_MAX in limits.h.

The declaration of getentropy in unistd.h and definition of
GETENTROPY_MAX in limits.h are currently conditional on
_NETBSD_SOURCE. When the next revision of POSIX is finalized, we can
expose them also under _POSIX_C_SOURCE > 20yymmL as usual -- and this
can be done as a pullup without breaking existing compiled programs.

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Add getentropy() to libc - a simple wrapper to access the kernel CSPRNG.

Posted to tech-userlevel@ a week ago and reviewed by riastradh@.

GETENTROPY(3) Library Functions Manual GETENTROPY(3)

Add getentropy() to libc - a simple wrapper to access the kernel CSPRNG.

Posted to tech-userlevel@ a week ago and reviewed by riastradh@.

GETENTROPY(3) Library Functions Manual GETENTROPY(3)

NAME
getentropy - fill a buffer with high quality random data

LIBRARY
Standard C Library (libc, -lc)

SYNOPSIS
#include <unistd.h>

int
getentropy(void *buf, size_t buflen);

DESCRIPTION
The getentropy() function fills a buffer with high quality random data,
suitable for seeding cryptographically secure psuedorandom number
generators.

getentropy() is only intended for seeding random number generators and is
not intended for use by regular code which simply needs secure random
data. For this purpose, please use arc4random(3).

The maximum value for buflen is 256 bytes.

IMPLEMENTATION NOTES
getentropy() reads from the sysctl(7) variable kern.arandom.

RETURN VALUES
The getentropy() function returns 0 on success, and -1 if an error
occurred.

ERRORS
getentropy() will succeed unless:

[EFAULT] The buf argument points to an invalid memory address.

[EIO] Too many bytes were requested.

SEE ALSO
arc4random(3), rnd(4)

STANDARDS
The getentropy() function is non-standard.

HISTORY
The getentropy() function first appeared in OpenBSD 5.6, then in
FreeBSD 12.0, and NetBSD 10.

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