224 lines
5.7 KiB
C
224 lines
5.7 KiB
C
/*
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* Copyright (C) 2015 Michael Brown <mbrown@fensystems.co.uk>.
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License as
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* published by the Free Software Foundation; either version 2 of the
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* License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful, but
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* WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
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* 02110-1301, USA.
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*
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* You can also choose to distribute this program under the terms of
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* the Unmodified Binary Distribution Licence (as given in the file
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* COPYING.UBDL), provided that you have satisfied its requirements.
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*/
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FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
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#include <errno.h>
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#include <ipxe/entropy.h>
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#include <ipxe/crc32.h>
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#include <ipxe/profile.h>
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#include <ipxe/efi/efi.h>
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#include <ipxe/efi/Protocol/Rng.h>
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/** @file
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*
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* EFI entropy source
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*
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*/
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/** Random number generator protocol */
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static EFI_RNG_PROTOCOL *efirng;
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EFI_REQUEST_PROTOCOL ( EFI_RNG_PROTOCOL, &efirng );
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/** Minimum number of bytes to request from RNG
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*
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* The UEFI spec states (for no apparently good reason) that "When a
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* Deterministic Random Bit Generator (DRBG) is used on the output of
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* a (raw) entropy source, its security level must be at least 256
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* bits." The EDK2 codebase (mis)interprets this to mean that the
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* call to GetRNG() should fail if given a buffer less than 32 bytes.
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*
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* Incidentally, nothing in the EFI RNG protocol provides any way to
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* report the actual amount of entropy returned by GetRNG().
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*/
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#define EFI_ENTROPY_RNG_LEN 32
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/** Time (in 100ns units) to delay waiting for timer tick
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*
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* In theory, UEFI allows us to specify a trigger time of zero to
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* simply wait for the next timer tick. In practice, specifying zero
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* seems to often return immediately, which produces almost no
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* entropy. Specify a delay of 1000ns to try to force an existent
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* delay.
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*/
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#define EFI_ENTROPY_TRIGGER_TIME 10
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/** Event used to wait for timer tick */
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static EFI_EVENT tick;
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/**
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* Enable entropy gathering
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*
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* @ret rc Return status code
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*/
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static int efi_entropy_enable ( void ) {
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EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
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EFI_STATUS efirc;
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int rc;
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DBGC ( &tick, "ENTROPY %s RNG protocol\n",
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( efirng ? "has" : "has no" ) );
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/* Create timer tick event */
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if ( ( efirc = bs->CreateEvent ( EVT_TIMER, TPL_NOTIFY, NULL, NULL,
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&tick ) ) != 0 ) {
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rc = -EEFI ( efirc );
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DBGC ( &tick, "ENTROPY could not create event: %s\n",
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strerror ( rc ) );
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return rc;
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}
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return 0;
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}
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/**
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* Disable entropy gathering
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*
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*/
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static void efi_entropy_disable ( void ) {
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EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
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/* Close timer tick event */
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bs->CloseEvent ( tick );
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}
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/**
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* Wait for a timer tick
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*
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* @ret low CPU profiling low-order bits, or negative error
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*/
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static int efi_entropy_tick ( void ) {
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EFI_BOOT_SERVICES *bs = efi_systab->BootServices;
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UINTN index;
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uint16_t low;
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EFI_STATUS efirc;
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int rc;
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/* Wait for next timer tick */
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if ( ( efirc = bs->SetTimer ( tick, TimerRelative,
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EFI_ENTROPY_TRIGGER_TIME ) ) != 0 ) {
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rc = -EEFI ( efirc );
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DBGC ( &tick, "ENTROPY could not set timer: %s\n",
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strerror ( rc ) );
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return rc;
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}
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if ( ( efirc = bs->WaitForEvent ( 1, &tick, &index ) ) != 0 ) {
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rc = -EEFI ( efirc );
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DBGC ( &tick, "ENTROPY could not wait for timer tick: %s\n",
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strerror ( rc ) );
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return rc;
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}
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/* Get current CPU profiling timestamp low-order bits */
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low = profile_timestamp();
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return low;
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}
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/**
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* Get noise sample from timer ticks
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*
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* @ret noise Noise sample
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* @ret rc Return status code
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*/
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static int efi_get_noise_ticks ( noise_sample_t *noise ) {
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int before;
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int after;
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int rc;
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/* Wait for a timer tick */
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before = efi_entropy_tick();
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if ( before < 0 ) {
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rc = before;
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return rc;
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}
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/* Wait for another timer tick */
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after = efi_entropy_tick();
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if ( after < 0 ) {
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rc = after;
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return rc;
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}
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/* Use TSC delta as noise sample */
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*noise = ( after - before );
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return 0;
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}
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/**
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* Get noise sample from RNG protocol
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*
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* @ret noise Noise sample
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* @ret rc Return status code
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*/
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static int efi_get_noise_rng ( noise_sample_t *noise ) {
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uint8_t buf[EFI_ENTROPY_RNG_LEN];
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EFI_STATUS efirc;
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int rc;
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/* Fail if we have no EFI RNG protocol */
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if ( ! efirng )
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return -ENOTSUP;
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/* Get the minimum allowed number of random bytes */
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if ( ( efirc = efirng->GetRNG ( efirng, NULL, EFI_ENTROPY_RNG_LEN,
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buf ) ) != 0 ) {
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rc = -EEFI ( efirc );
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DBGC ( &tick, "ENTROPY could not read from RNG: %s\n",
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strerror ( rc ) );
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return rc;
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}
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/* Reduce random bytes to a single noise sample. This seems
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* like overkill, but we have no way of knowing how much
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* entropy is actually present in the bytes returned by the
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* RNG protocol.
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*/
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*noise = crc32_le ( 0, buf, sizeof ( buf ) );
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return 0;
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}
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/**
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* Get noise sample
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*
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* @ret noise Noise sample
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* @ret rc Return status code
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*/
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static int efi_get_noise ( noise_sample_t *noise ) {
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int rc;
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/* Try RNG first, falling back to timer ticks */
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if ( ( ( rc = efi_get_noise_rng ( noise ) ) != 0 ) &&
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( ( rc = efi_get_noise_ticks ( noise ) ) != 0 ) )
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return rc;
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return 0;
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}
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PROVIDE_ENTROPY_INLINE ( efi, min_entropy_per_sample );
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PROVIDE_ENTROPY ( efi, entropy_enable, efi_entropy_enable );
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PROVIDE_ENTROPY ( efi, entropy_disable, efi_entropy_disable );
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PROVIDE_ENTROPY ( efi, get_noise, efi_get_noise );
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