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Kernel

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The kernel of iOS is the XNU kernel. Pre-2.0, it was vulnerable to the Ramdisk Hack and may still be, but iBoot doesn't allow boot-args to be passed anymore. It is mapped to memory at 0x80000000, forcing a 2/2GB address separation, similar to Windows 32-bit model. On older iOS the separation was 3/1 (mapping the kernel at 0xC0000000), closer to the Linux model.

Note, that this is NOT like 32-bit OS X, wherein the kernel resides in its own address space, but more like OS X 64-bit, wherein CR3 is shared (albeit an address space larger by several orders of magnitude). See the appropriate section

ASLR

Main article: Kernel ASLR

As of iOS 6, the kernel is subject to ASLR, much akin to Mountain Lion (OS X 10.8). This make exploitation harder as the location of kernel code cannot be known.

On production devices, the kernel is always stored as a statically linked cache stored at /System/Library/Caches/com.apple.kernelcaches/kernelcache that is decompressed and run on startup. On development devices the kernel is stored in the same location as OS X, at /mach_kernel.

Stack

The kernel maintains its stack at 0xd2000000.

Boot-Args

Like its OS X counterpart, iOS's XNU accepts command line arguments (though the actual passing of arguments is done by iBoot, which as of late refuses to do so). Arguments may be directed at the kernel proper, or any one of the many KExts (discussed below). The arguments of the kernel are largely the same as those of OS X.

KExts use boot-args as well, as can be seen when disassembly by calls to PE_parse_boot_argn (usually exported, @0x80240800 on the iOS 5 iPod 4g kernel). Finding references (using IDA) reveals hundreds places in the code wherein arguments are parsed in modules, pertaining to Flash, HDMI, and AMFI.

Here's a list of boot-args extracted with the IDA script by MuscleNerd: 

_nand-part-poison
_panicd_corename
_panicd_ip
_router_ip
acc_debug
aesdev
als_enable_debug
amfi
amfi_allow_any_signature
amfi_get_out_of_my_way
amfi_unrestrict_task_for_pid
AppleEmbeddedUSBArbitrator-debug
AppleS5L8930XUSBArbitrator-debug
AppleUSBPhy-debug
arm7m-enable-jtag
-b
backlight-level
backlight-logging
baseband-spi-sclk-period
bcom.chip.driveStrength_mA
bcom.chip.watermark
bcom.clock.sd-rate
bcom.devif.fn2-block-size
bcom.devif.rx-retries
bcom.devif.transaction-log
bcom.devif.tx-retries
bcom.feature.flags
bcom.ps.inactivity.timeout
bcom.wte.thread-priority
boot-uuid
brightness
burnin-size
cameraclocks
charger-debug
cpus
cs_debug
cs_enforcement_disable
darkwake
dart
dcc
debug
disable-usb-iap
dp_async_event_fail_hard
dp_audio_driver_level
dp_audio_driver_mask
dp_audio_interface_level
dp_audio_interface_mask
dp_controller_level
dp_controller_mask
dp_device_level
dp_device_mask
dp_display_interface_level
dp_display_interface_mask
dp_interface_level
dp_interface_mask
dp_log_level
dp_max_channel_count_lpcm
dp_max_sample_rate_lpcm
dp_max_sample_size_lpcm
dp_min_channel_count_lpcm
dp_min_sample_rate_lpcm
dp_min_sample_size_lpcm
dp_service_level
dp_service_mask
dpsm
dvb
dvc
dvd
effaceable-enable-full-scan
effaceable-enable-wipe
enable-acsleep
fairshare_minblockedtime
fill
fixedpriority_quantum
fix-parity
force-usb-host
force-usb-power
hdmi_max_channel_count_lpcm
hdmi_max_sample_rate_lpcm
hdmi_max_sample_size_lpcm
hdmi_min_channel_count_lpcm
hdmi_min_sample_rate_lpcm
hdmi_min_sample_size_lpcm
hdmi_protection_type
hp-detect-invert
hp-pop-workaround
hp-switch-force-config
hp-switch-ramp
hsic
i2c-logsize
i2c-verbose
ifa_debug
ifnet_debug
initmcl
io
iopfmi-timeout
iotrace
jpeg-log
jtag
kdp_crashdump_pkt_size
kdp_ip_addr
kdp_match_mac
kdp_match_name
keepsyms
kextlog
link_recovery_enabled
mbuf_debug
mbuf_pool
mcache_flags
mleak_sample_factor
mseg
msgbuf
mt-bytes
mt-strings
mtxspin
nand-boot-malloc
nand-check-vs
nand-commands
nand-disable-driver
nand-dump-vs-table
nand-enable-adm
nand-enable-reformat
nand-enable-yaftl
nand-erase
nand-erase-install
nand-fbbt-publish
nand-force-restore
nand-idle-timeout-ms
nand-ignore-ptab
nand-index-cache-size
nand-latency-us
nand-max-pages
nand-neuralize
nand-nvram-debug
nand-ppn-debug
nand-ppn-vs-debug
nand-qual
nand-queue-entries
nand-read-blocks-max
nand-read-dccycle-clks
nand-read-hold-clks
nand-readonly
nand-read-setup-clks
nand-reorder-defer-max
nand-reorder-defer-size-trigger
nand-reorder-read-promote-max
nand-reset-burnin
nand-save-rma-data
nand-set-rma
nand-sftl-cache-drain
nand-sleep-debug-panic
nand-slow-timings
nand-wearlevel-timeout-ms
nand-whiten-metadata
nand-wipe
nand-write-blocks-max
nand-write-hold-clks
nand-write-setup-clks
nbuf
ncl
net.inet6.ip6.scopedroute
net_affinity
net_rtref
network-type
-no64exec
-novfscache
panicd_port
pcp
pctb
pdmvr
pio-error
pmu-chargetrap
pmu-debug
ppn-clean
-progress
prox_enable_debug
pthtest
rd
remote_nmi
rootdev
-s
sdio.clock.base-rate
sdio.clock.sd-rate
sdio.debug.abort-init
sdio.debug.init-delay
sdio.log.flags
sdio.log.level
sdio.transfer.max-pio-blocks
sdio.transfer.max-pio-size
sdio.transfer.mode
serial
sgx_panic_on_recovery
shadev
slto_us
socket_debug
torchcltm0
usb
usb_dev_nmi
usb_dev_reset
-vnode_cache_defeat
wdt
wfi
wlan.ap.channel
wlan.debug.abort-init
wlan.debug.generate-mac
wlan.log.flags
wlan.log.level
wlan.log.timestamp
wlan.netmanager.stats-timer-interval
wlan.panic.factory
wqsize
WTE
-x

Versions

iOS has consistently maintained a higher kernel version than the corresponding version of OS X. At the time of writing, OS X Mountain Lion's XNU is 20xx, whereas iOS is 21xx. This is not surprising, considering that iOS has novel features (such as Kernel ASLR, the default freezer, and various security hardening features) which are first incorporated in it, and only later make it to OS X. The following demonstrates the two OS versions at present:

OS X Mountain Lion: 

Darwin Kernel Version 12.2.0: Sat Aug 25 00:48:52 PDT 2012; root:xnu-2050.18.24~1/RELEASE_X86_64 x86_64

iOS 6.1: 

Darwin Kernel Version 13.0.0: Sun Dec 16 20:01:39 PST 2012; root:xnu-2107.7.55~11/RELEASE_ARM_S5L8950X

Note: The RELEASE_ARM_xxxxxxxx file obviously differs on device / CPU.

Version Build Comment
1A420 Darwin Kernel Version 4.4.2-Purple-19: Thu Mar 8 01:43:04 PST 2007; root:xnu-933.0.14~46/RELEASE_ARM_S5L8900XRB from prototype - not sure if 100% correct.
1.0.0
1.0.1
1.0.2
1.1.1
1.1.2
1.1.3
1.1.4
2.0
2.0.1
2.0.2
2.1
2.2
2.2.1
3.0
3.0.1
3.1
3.1.2
3.1.3
3.2
3.2.1
3.2.2
4.0
4.0.1
4.0.2
4.1
4.2.1 Darwin Kernel Version 10.4.0: Wed Oct 20 20:14:45 PDT 2010; root:xnu-1504.58.28~3/RELEASE_ARM_S5L8930X
4.3
4.3.1
4.3.2
4.3.3 Darwin Kernel Version 11.0.0: Wed Mar 30 18:44:45 PDT 2011; root:xnu-1735.46~10/RELEASE_ARM_S5L8920X
4.3.4
4.3.5 Darwin Kernel Version 11.0.0: Sat Jul 9 00:59:43 PDT 2011; root:xnu-1735.47~1/RELEASE_ARM_S5L8930X
5.0 Darwin Kernel Version 11.0.0: Thu Sep 15 23:34:43 PDT 2011; root:xnu-1878.4.43~2/RELEASE_ARM_S5L8940X
5.0.1 Darwin Kernel Version 11.0.0: Tue Nov 1 20:34:16 PDT 2011; root:xnu-1878.4.46~1/RELEASE_ARM_S5L8940X
5.1 Darwin Kernel Version 11.0.0: Wed Feb 1 23:18:07 PST 2012; root:xnu-1878.11.8~1/RELEASE_ARM_S5L8945X
5.1.1 Darwin Kernel Version 11.0.0: Sun Apr 8 21:51:26 PDT 2012; root:xnu-1878.11.10~1/RELEASE_ARM_S5L8930X
6.0 Darwin Kernel Version 13.0.0: Sun Aug 19 00:31:06 PDT 2012; root:xnu-2107.2.33~4/RELEASE_ARM_S5L8950X
6.0.1 Darwin Kernel Version 13.0.0: Wed Oct 10 23:29:02 PDT 2012; root:xnu-2107.2.34~2/RELEASE_ARM_S5L8930X
6.0.2 Darwin Kernel Version 13.0.0: Wed Oct 10 23:32:19 PDT 2012; root:xnu-2107.2.34~2/RELEASE_ARM_S5L8950X
6.1 Darwin Kernel Version 13.0.0: Sun Dec 16 20:01:39 PST 2012; root:xnu-2107.7.55~11/RELEASE_ARM_S5L8950X
6.1.1 Darwin Kernel Version 13.0.0: Sun Dec 16 19:58:44 PST 2012; root:xnu-2107.7.55~11/RELEASE_ARM_S5L8940X

Source Code

As XNU is based off of the BSD kernel, it is open source. The source is under a 3-clause BSD License for the original BSD portions with the portions added by Apple under the Apple Public Source License. The versions contained in iOS are not available, instead only versions used in OS X are available. This does not appear to be legal as per §2.3 in the APSL: 

2.3     Distribution of Executable Versions.  In addition, if You Externally Deploy Covered
Code (Original Code and/or Modifications) in object code, executable form only, You must
include a prominent notice, in the code itself as well as in related documentation, stating
that Source Code of the Covered Code is available under the terms of this License with
information on how and where to obtain such Source Code.

with Source Code defined in §1.8: 

1.8     "Source Code" means the human readable form of a program or other work that is
suitable for making modifications to it, including all modules it contains, plus any
associated interface definition files, scripts used to control compilation and installation
of an executable (object code).

It is worth noting that Apple does not list XNU as being an open source component of iOS. This can be seen by viewing opensource.apple.com and selecting any iOS version. As far as can be told, none of the versions of XNU are available in source version.

There are many other open souce components that iOS uses that are not listed, such as:

It does not appear that Apple assumes what you see in the OS X pages are also on iOS as JavaScriptCore, WebCore, among others are listed on both OS X (10.8) and iOS (6.0), albeit different versions.

It is also worth noting that gdb (GCC debugger) and ld64 are listed as components in iOS 6.0. Why there are present is a mystery as they are not present on unaltered devices, but only through Cydia or Xcode's DeveloperImage.dmg.

Kernel Extensions

iOS, sadly, does not have kexts floating around the file system, but they are indeed present. The kernelcache can be unpacked to show the kernel proper, along with the kexts (all packed in the __PRELINK_TEXT section) and their plists (in the __PRELINK_INFO section).

The Cydia supplied kextstat does not work on iOS. Sadly, the reason is that kextstat relies on kmod_get_info(...), which is a deprecated (and recently removed) API in recent iOS and OS X versions. With that said, the kexts do exist. The alternative, jkextstat, does work on recent iOS versions. jkextstat can cause some confusion as it uses the executable name kextstat, similar to how calling g++ just launches gcc but with parameters to treat all .c files as C++ files.

The following is the output from jkextstat on an iPod touch 4G running iOS 6(?): 

Podicum:~ root# ./kextstat 
  0 __kernel__ 
  1 kpi.bsd 
  2 kpi.dsep 
  3 kpi.iokit 
  4 kpi.libkern 
  5 kpi.mach 
  6 kpi.private 
  7 kpi.unsupported 
  8 driver.AppleARMPlatform <1 3 4 5 6 7>
  9 iokit.IOStorageFamily <1 3 4 5 6 7>
 10 driver.DiskImages <1 3 4 5 6 7 9>
 11 driver.FairPlayIOKit <1 3 4 5 6 7>
 12 driver.IOSlaveProcessor <3 4>
 13 driver.IOP_s5l8930x_firmware <3 4 12>
 14 iokit.AppleProfileFamily <1 3 4 5 6 7>
 15 iokit.IOCryptoAcceleratorFamily <1 3 4 5 7>
 16 driver.AppleMobileFileIntegrity <1 2 3 4 5 6 7 15>
 17 iokit.IONetworkingFamily <1 3 4 5 6 7>
 18 iokit.IOUserEthernet <1 3 4 5 6 16 17>
 19 platform.AppleKernelStorage <3 4 7>
 20 iokit.IOSurface <1 3 4 5 6 7 8>
 21 iokit.IOStreamFamily <3 4 5>
 22 iokit.IOAudio2Family <1 3 4 5 21>
 23 driver.AppleAC3Passthrough <1 3 4 5 7 8 11 21 22>
 24 iokit.EncryptedBlockStorage <1 3 4 5 9 15>
 25 iokit.IOFlashStorage <1 3 4 5 7 9 24>
 26 driver.AppleEffaceableStorage <1 3 4 5 7 8 25>
 27 driver.AppleKeyStore <1 3 4 5 6 7 15 16 26>
 28 kext.AppleMatch <1 4>
 29 security.sandbox <1 2 3 4 5 6 7 16 28>
 30 driver.AppleS5L8930X <1 3 4 5 7 8>
 31 iokit.IOHIDFamily <1 3 4 5 6 7 16>
 32 driver.AppleM68Buttons <1 3 4 5 7 8 31>
 33 iokit.IOUSBDeviceFamily <1 3 4 5>
 34 iokit.IOSerialFamily <1 3 4 5 6 7>
 35 driver.AppleOnboardSerial <1 3 4 5 7 34>
 36 iokit.IOAccessoryManager <3 4 5 7 8 33 34 35>
 37 driver.AppleProfileTimestampAction <1 3 4 5 14>
 38 driver.AppleProfileThreadInfoAction <1 3 4 6 14>
 39 driver.AppleProfileKEventAction <1 3 4 14>
 40 driver.AppleProfileRegisterStateAction <1 3 4 14>
 41 driver.AppleProfileCallstackAction <1 3 4 5 6 14>
 42 driver.AppleProfileReadCounterAction <3 4 6 14>
 43 driver.AppleARMPL192VIC <3 4 5 7 8>
 44 driver.AppleCDMA <1 3 4 5 7 8 15>
 45 driver.IODARTFamily <3 4 5>
 46 driver.AppleS5L8930XDART <1 3 4 5 7 8 45>
 47 iokit.IOSDIOFamily <1 3 4 5 7>
 48 driver.AppleIOPSDIO <1 3 4 5 7 8 12 47>
 49 driver.AppleIOPFMI <1 3 4 5 7 8 12 25>
 50 driver.AppleSamsungSPI <1 3 4 5 7 8>
 51 driver.AppleSamsungSerial <1 3 4 5 7 8 34 35>
 52 driver.AppleSamsungPKE <3 4 5 7 8 15>
 53 driver.AppleS5L8920X <1 3 4 5 7 8>
 54 driver.AppleSamsungI2S <1 3 4 5 7 8>
 55 driver.AppleEmbeddedUSB <1 3 4 5 7 8>
 56 driver.AppleS5L8930XUSBPhy <1 3 4 5 7 8 55>
 57 iokit.IOUSBFamily <1 3 4 5 7>
 58 driver.AppleUSBEHCI <1 3 4 5 7 57>
 59 driver.AppleUSBComposite <1 3 4 57>
 60 driver.AppleEmbeddedUSBHost <1 3 4 5 7 55 57 59>
 61 driver.AppleUSBOHCI <1 3 4 5 57>
 62 driver.AppleUSBOHCIARM <3 4 5 8 55 57 60 61>
 63 driver.AppleUSBHub <1 3 4 5 57>
 64 driver.AppleUSBEHCIARM <3 4 5 8 55 57 58 60 63>
 65 driver.AppleS5L8930XUSB <1 3 4 5 7 8 55 57 58 60 61 62 64>
 66 driver.AppleARM7M <3 4 8 12>
 67 driver.EmbeddedIOP <3 4 5 12>
 68 driver.AppleVXD375 <1 3 4 5 7 8 11>
 69 driver.AppleD1815PMU <1 3 4 5 7 8 31>
 70 iokit.AppleARMIISAudio <1 3 4 5 7 22>
 71 driver.AppleEmbeddedAudio <1 3 4 5 7 8 22 31 70>
 72 driver.AppleCS42L59Audio <3 4 5 8 22 31 70 71>
 73 driver.AppleEmbeddedAccelerometer <3 4 5 7 8 31>
 74 driver.AppleEmbeddedGyro <1 3 4 5 7 8 31>
 75 driver.AppleEmbeddedLightSensor <3 4 5 7 8 31>
 76 iokit.IOAcceleratorFamily <1 3 4 5 7 8>
 77 IMGSGX535 <1 3 4 5 7 8 76>
 78 driver.H2H264VideoEncoderDriver <1 3 4 5 7 8>
 79 driver.AppleJPEGDriver <1 3 4 5 7 8>
 80 driver.AppleH3CameraInterface <1 3 4 5 7 8>
 81 driver.AppleM2ScalerCSCDriver <1 3 4 5 7 8 45>
 82 iokit.IOMobileGraphicsFamily <1 3 4 5 7 8>
 83 driver.AppleDisplayPipe <1 3 4 5 7 8 82>
 84 driver.AppleCLCD <1 3 4 5 7 8 82 83>
 85 driver.AppleSamsungMIPIDSI <1 3 4 5 7 8>
 86 driver.ApplePinotLCD <1 3 4 5 7 8>
 87 driver.AppleSamsungSWI <1 3 4 5 7 8>
 88 iokit.IODisplayPortFamily <1 3 4 5 6 7 22>
 89 driver.AppleRGBOUT <1 3 4 5 7 8 82 83 88>
 90 driver.AppleTVOut <1 3 4 5 7 8>
 91 driver.AppleAMC_r2 <1 3 4 5 7 8 11 21 22>
 92 driver.AppleSamsungDPTX <3 4 5 7 8 88>
 93 driver.AppleSynopsysOTGDevice <1 3 4 5 7 8 33 55>
 94 driver.AppleNANDFTL <1 3 4 5 7 9 25>
 95 driver.AppleNANDLegacyFTL <1 3 4 5 9 25 94>
 96 AppleFSCompression.AppleFSCompressionTypeZlib <1 2 3 4 6>
 97 IOTextEncryptionFamily <1 3 4 5 7 11>
 98 driver.AppleBSDKextStarter <3 4>
 99 nke.ppp <1 3 4 5 6 7>
100 nke.l2tp <1 3 4 5 6 7 99>
101 nke.pptp <1 3 4 5 6 7 99>
102 iokit.IO80211Family <1 3 4 5 6 7 17>
103 driver.AppleBCMWLANCore <1 3 4 5 6 7 8 17 102>
104 driver.AppleBCMWLANBusInterfaceSDIO <1 3 4 5 6 7 8 47 103>
105 driver.AppleDiagnosticDataAccessReadOnly <1 3 4 5 7 8 94>
106 driver.LightweightVolumeManager <1 3 4 5 9 15 24 26>
107 driver.IOFlashNVRAM <1 3 4 5 6 7 25>
108 driver.AppleNANDFirmware <1 3 4 5 25>
109 driver.AppleImage3NORAccess <1 3 4 5 7 8 15 108>
110 driver.AppleBluetooth <1 3 4 5 7 8>
111 driver.AppleMultitouchSPI <1 3 4 5 7 8>
112 driver.AppleUSBMike <1 3 4 5 8 22 33>
113 driver.AppleUSBDeviceMux <1 3 4 5 6 7 33>
114 driver.AppleUSBEthernetDevice <1 3 4 5 6 8 17 33>

For a specific extension, e.g. SandBox, the full information (including the handy load address) is also accessible:

root# ./jkextstat -b sandbox -x: 

<plist>
<dict>
        <key>CFBundleIdentifier</key>
        <string>com.apple.security.sandbox</string>
        <key>CFBundleVersion</key>
        <string>154.7</string>
        <key>OSBundleCPUSubtype</key>
        <integer>9</integer>
        <key>OSBundleCPUType</key>
        <integer>12</integer>
        <key>OSBundleDependencies</key>
        <array>
                <integer>6</integer>
                <integer>7</integer>
                <integer>5</integer>
                <integer>3</integer>
                <integer>28</integer>
                <integer>1</integer>
                <integer>4</integer>
                <integer>16</integer>
                <integer>2</integer>
        </array>
        <key>OSBundleExecutablePath</key>
        <string>/System/Library/Extensions/Sandbox.kext/Sandbox</string>
        <key>OSBundleIsInterface</key>
        <false/>
        <key>OSBundleLoadAddress</key>
        <integer>2153734144</integer>
        <key>OSBundleLoadSize</key>
        <integer>36864</integer>
        <key>OSBundleLoadTag</key>
        <integer>29</integer>
        <key>OSBundleMachOHeaders</key>
        
        zvrt/gwAAAAJAAAACwAAAAMAAAAgAgAAAQAAAAEAAAAEAQAAX19URVhUAAAAAAAAAAAA
        AABgX4AAgAAAAAAAAACAAAAHAAAABwAAAAMAAAAAAAAAX190ZXh0AAAAAAAAAAAAAF9f
        VEVYVAAAAAAAAAAAAADMbV+AKGEAAMwNAAACAAAAAAAAAAAAAAAABwCAAAAAAAAAAABf
        X2NzdHJpbmcAAAAAAAAAX19URVhUAAAAAAAAAAAAAPTOX4DLDQAA9G4AAAAAAAAAAAAA
        AAAAAAIAAAAAAAAAAAAAAF9fY29uc3QAAAAAAAAAAABfX1RFWFQAAAAAAAAAAAAAwNxf
        gDEDAADAfAAABAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQAAAAQBAABfX0RBVEEAAAAA
        AAAAAAAAAOBfgAAQAAAAgAAAABAAAAcAAAAHAAAAAwAAAAAAAABfX2RhdGEAAAAAAAAA
        AAAAX19EQVRBAAAAAAAAAAAAAADgX4C0BgAAAIAAAAQAAAAAAAAAAAAAAAAAAAAAAAAA
        AAAAAF9fYnNzAAAAAAAAAAAAAABfX0RBVEEAAAAAAAAAAAAAwOZfgHgAAAAAAAAABAAA
        AAAAAAAAAAAAAQAAAAAAAAAAAAAAX19jb21tb24AAAAAAAAAAF9fREFUQQAAAAAAAAAA
        AAA451+AGAAAAAAAAAACAAAAAAAAAAAAAAABAAAAAAAAAAAAAAAbAAAAGAAAABasg7Y2
        TzkVrtqsgOViBQ0=
        
        <key>OSBundlePath</key>
        <string>/System/Library/Extensions/Sandbox.kext</string>
        <key>OSBundlePrelinked</key>
        <true/>
        <key>OSBundleRetainCount</key>
        <integer>0</integer>
        <key>OSBundleStarted</key>
        <true/>
        <key>OSBundleUUID</key>
        
        FqyDtjZPORWu2qyA5WIFDQ==
        
        <key>OSBundleWiredSize</key>
        <integer>36864</integer>
        <key>OSKernelResource</key>
        <false/>
</dict>
</plist>


It's also worth mentioning that, in the above listing, the OSBundleMachOHeaders (base-64 encoded binary headers) leak kernel addresses in iOS 6.0, defeating Kernel ASLR. This has been quickly fixed in iOS 6.0.1, effectively locking down iOS for the foreseeable future, thanks to security researcher mdowd.

See Also

External Links

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