postgres/src/backend/utils/misc/backend_random.c

/*-------------------------------------------------------------------------
 *
 * backend_random.c
 *	  Backend random number generation routine.
 *
 * pg_backend_random() function fills a buffer with random bytes. Normally,
 * it is just a thin wrapper around pg_strong_random(), but when compiled
 * with --disable-strong-random, we provide a built-in implementation.
 *
 * This function is used for generating nonces in authentication, and for
 * random salt generation in pgcrypto. The built-in implementation is not
 * cryptographically strong, but if the user asked for it, we'll go ahead
 * and use it anyway.
 *
 * The built-in implementation uses the standard erand48 algorithm, with
 * a seed shared between all backends.
 *
 * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  src/backend/utils/misc/backend_random.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <sys/time.h>

#include "miscadmin.h"
#include "storage/lwlock.h"
#include "storage/shmem.h"
#include "utils/backend_random.h"
#include "utils/timestamp.h"

#ifdef HAVE_STRONG_RANDOM

Size
BackendRandomShmemSize(void)
{
	return 0;
}

void
BackendRandomShmemInit(void)
{
	/* do nothing */
}

bool
pg_backend_random(char *dst, int len)
{
	/* should not be called in postmaster */
	Assert (IsUnderPostmaster || !IsPostmasterEnvironment);

	return pg_strong_random(dst, len);
}

#else

/*
 * Seed for the PRNG, stored in shared memory.
 *
 * Protected by BackendRandomLock.
 */
typedef struct
{
	bool		initialized;
	unsigned short seed[3];
} BackendRandomShmemStruct;

static BackendRandomShmemStruct *BackendRandomShmem;

Size
BackendRandomShmemSize(void)
{
	return sizeof(BackendRandomShmemStruct);
}

void
BackendRandomShmemInit(void)
{
	bool		found;

	BackendRandomShmem = (BackendRandomShmemStruct *)
		ShmemInitStruct("Backend PRNG state",
						BackendRandomShmemSize(),
						&found);

	if (!IsUnderPostmaster)
	{
		Assert(!found);

		BackendRandomShmem->initialized = false;
	}
	else
		Assert(found);
}

bool
pg_backend_random(char *dst, int len)
{
	int			i;
	char	   *end = dst + len;

	/* should not be called in postmaster */
	Assert (IsUnderPostmaster || !IsPostmasterEnvironment);

	LWLockAcquire(BackendRandomLock, LW_EXCLUSIVE);

	/*
	 * Seed the PRNG on the first use.
	 */
	if (!BackendRandomShmem->initialized)
	{
		struct timeval now;

		gettimeofday(&now, NULL);

		BackendRandomShmem->seed[0] = now.tv_sec;
		BackendRandomShmem->seed[1] = (unsigned short) (now.tv_usec);
		BackendRandomShmem->seed[2] = (unsigned short) (now.tv_usec >> 16);

		/*
		 * Mix in the cancel key, generated by the postmaster. This adds
		 * what little entropy the postmaster had to the seed.
		 */
		BackendRandomShmem->seed[0] ^= (MyCancelKey);
		BackendRandomShmem->seed[1] ^= (MyCancelKey >> 16);

		BackendRandomShmem->initialized = true;
	}

	for (i = 0; dst < end; i++)
	{
		uint32		r;
		int			j;

		/*
		 * pg_jrand48 returns a 32-bit integer. Fill the next 4 bytes from it.
		 */
		r  = (uint32) pg_jrand48(BackendRandomShmem->seed);

		for (j = 0; j < 4 && dst < end; j++)
		{
			*(dst++) = (char) (r & 0xFF);
			r >>= 8;
		}
	}
	LWLockRelease(BackendRandomLock);

	return true;
}


#endif /* HAVE_STRONG_RANDOM */