Re: [syzbot] BUG: Address NUM::fb on device lo is missing its host route.

[syzbot <syzbot+57f410c9a4f8d7a441d6@syzkaller.appspotmail.com>]

Hello,

syzbot tried to test the proposed patch but the build/boot failed:

SYZFAIL: failed to recv rpc

SYZFAIL: failed to recv rpc
fd=3 want=4 recv=0 n=0 (errno 9: Bad file descriptor)
execution of simple program fails: NOTFAIL: arptable checkpoint: getsockopt(ARPT_SO_GET_ENTRIES) failed. table=filter (errno 22: Invalid argument).


Warning: Permanently added '[localhost]:25612' (ED25519) to the list of known hosts.
2026/09/28 15:31:20 parsed 1 programs
2026/09/28 15:31:20 serving rpc on tcp://36177
[   47.553227][ T5746] cgroup: Unknown subsys name 'net'
[   47.698693][ T5746] cgroup: Unknown subsys name 'cpuset'
[   47.702860][ T5746] cgroup: Unknown subsys name 'rlimit'
Setting up swapspace version 1, size = 127995904 bytes
[   48.856439][ T5746] Adding 124996k swap on ./swap-file.  Priority:0 extents:1 across:124996k 
[   51.171997][ T5752] soft_limit_in_bytes is deprecated and will be removed. Writing any value to this file has no effect. Please report your usecase to linux-mm@kvack.org if you depend on this functionality.
[   51.176597][ T5752] ebtables: xtables 32bit compat interface no longer supported in namespaces and will be removed soon.
[   51.187163][ T5752] arp_tables: xtables 32bit compat interface no longer supported in namespaces and will be removed soon.
[   51.584025][   T46] wlan0: Created IBSS using preconfigured BSSID 50:50:50:50:50:50
[   51.584041][   T46] wlan0: Creating new IBSS network, BSSID 50:50:50:50:50:50
[   51.638990][   T38] wlan1: Created IBSS using preconfigured BSSID 50:50:50:50:50:50
[   51.639014][   T38] wlan1: Creating new IBSS network, BSSID 50:50:50:50:50:50
[   52.584253][ T5769] bridge0: port 1(bridge_slave_0) entered blocking state
[   52.584749][ T5769] bridge0: port 1(bridge_slave_0) entered disabled state
[   52.584809][ T5769] bridge_slave_0: entered allmulticast mode
[   52.586142][ T5769] bridge_slave_0: entered promiscuous mode
[   52.588800][ T5769] bridge0: port 2(bridge_slave_1) entered blocking state
[   52.588842][ T5769] bridge0: port 2(bridge_slave_1) entered disabled state
[   52.588899][ T5769] bridge_slave_1: entered allmulticast mode
[   52.589826][ T5769] bridge_slave_1: entered promiscuous mode
[   52.609061][ T5769] bond0: (slave bond_slave_0): Enslaving as an active interface with an up link
[   52.612156][ T5769] bond0: (slave bond_slave_1): Enslaving as an active interface with an up link
[   52.634772][ T5769] team0: Port device team_slave_0 added
[   52.637853][ T5769] team0: Port device team_slave_1 added
[   52.659595][ T5769] batman_adv: batadv0: Adding interface: batadv_slave_0
[   52.659612][ T5769] batman_adv: batadv0: The MTU of interface batadv_slave_0 is too small (1500) to handle the transport of batman-adv packets. Packets going over this interface will be fragmented on layer2 which could impact the performance. Setting the MTU to 1532 would solve the problem.
[   52.659634][ T5769] batman_adv: batadv0: Not using interface batadv_slave_0 (retrying later): interface not active
[   52.661967][ T5769] batman_adv: batadv0: Adding interface: batadv_slave_1
[   52.661981][ T5769] batman_adv: batadv0: The MTU of interface batadv_slave_1 is too small (1500) to handle the transport of batman-adv packets. Packets going over this interface will be fragmented on layer2 which could impact the performance. Setting the MTU to 1532 would solve the problem.
[   52.662001][ T5769] batman_adv: batadv0: Not using interface batadv_slave_1 (retrying later): interface not active
[   52.698031][ T5769] hsr_slave_0: entered promiscuous mode
[   52.698964][ T5769] hsr_slave_1: entered promiscuous mode
[   52.831553][ T5769] netdevsim netdevsim5 netdevsim0: renamed from eth0
[   52.838068][ T5769] 8021q: adding VLAN 0 to HW filter on device netdevsim0
[   52.839020][ T5769] netdevsim netdevsim5 netdevsim1: renamed from eth1
[   52.847513][ T5769] 8021q: adding VLAN 0 to HW filter on device netdevsim1
[   52.848454][ T5769] netdevsim netdevsim5 netdevsim2: renamed from eth2
[   52.853584][ T5769] 8021q: adding VLAN 0 to HW filter on device netdevsim2
[   52.854492][ T5769] netdevsim netdevsim5 netdevsim3: renamed from eth3
[   52.860087][ T5769] 8021q: adding VLAN 0 to HW filter on device netdevsim3
[   52.924459][ T5769] 8021q: adding VLAN 0 to HW filter on device bond0
[   52.939304][ T5769] 8021q: adding VLAN 0 to HW filter on device team0
[   52.944711][   T74] bridge0: port 1(bridge_slave_0) entered blocking state
[   52.945117][   T74] bridge0: port 1(bridge_slave_0) entered forwarding state
[   52.953389][   T74] bridge0: port 2(bridge_slave_1) entered blocking state
[   52.953436][   T74] bridge0: port 2(bridge_slave_1) entered forwarding state
[   53.267244][ T5769] 8021q: adding VLAN 0 to HW filter on device batadv0
[   53.432851][ T5769] veth0_vlan: entered promiscuous mode
[   53.438431][ T5769] veth1_vlan: entered promiscuous mode
[   53.457016][ T5769] veth0_macvtap: entered promiscuous mode
[   53.460801][ T5769] veth1_macvtap: entered promiscuous mode
[   53.473078][ T5769] batman_adv: batadv0: Interface activated: batadv_slave_0
[   53.480407][ T5769] batman_adv: batadv0: Interface activated: batadv_slave_1
[   53.509123][   T12] netdevsim netdevsim5 netdevsim3: set [1, 0] type 2 family 0 port 6081 - 0
[   53.566894][   T12] netdevsim netdevsim5 netdevsim3 (unregistering): unset [1, 0] type 2 family 0 port 6081 - 0
[   53.575956][ T5761] netdevsim netdevsim5 netdevsim1: set [1, 0] type 2 family 0 port 6081 - 0
[   53.655132][ T5761] netdevsim netdevsim5 netdevsim0: set [1, 0] type 2 family 0 port 6081 - 0
[   53.674478][   T12] netdevsim netdevsim5 netdevsim1 (unregistering): unset [1, 0] type 2 family 0 port 6081 - 0
[   53.689000][ T5818] Bluetooth: hci0: unexpected cc 0x0c03 length: 249 > 1
[   53.689955][ T5818] Bluetooth: hci0: unexpected cc 0x1003 length: 249 > 9
[   53.690417][ T5818] Bluetooth: hci0: unexpected cc 0x1001 length: 249 > 9
[   53.691659][ T5818] Bluetooth: hci0: unexpected cc 0x0c23 length: 249 > 4
[   53.692196][ T5818] Bluetooth: hci0: unexpected cc 0x0c38 length: 249 > 2
[   53.737071][   T12] netdevsim netdevsim5 netdevsim0 (unregistering): unset [1, 0] type 2 family 0 port 6081 - 0
[   54.878528][ T5840] ip_tables: xtables 32bit compat interface no longer supported in namespaces and will be removed soon.
[   54.878912][ T5840] ip6_tables: xtables 32bit compat interface no longer supported in namespaces and will be removed soon.
[   55.016366][   T12] bridge_slave_1: left allmulticast mode
[   55.016404][   T12] bridge_slave_1: left promiscuous mode
[   55.017015][   T12] bridge0: port 2(bridge_slave_1) entered disabled state
[   55.021736][   T12] bridge_slave_0: left allmulticast mode
[   55.021756][   T12] bridge_slave_0: left promiscuous mode
[   55.021918][   T12] bridge0: port 1(bridge_slave_0) entered disabled state
[   55.161216][   T12] bond0 (unregistering): (slave bond_slave_0): Releasing backup interface
[   55.163318][   T12] bond0 (unregistering): (slave bond_slave_1): Releasing backup interface
[   55.164136][   T12] bond0 (unregistering): Released all slaves
[   55.256269][   T12] hsr_slave_0: left promiscuous mode
[   55.258264][   T12] hsr_slave_1: left promiscuous mode
[   55.258734][   T12] batman_adv: batadv0: Interface deactivated: batadv_slave_0
[   55.258752][   T12] batman_adv: batadv0: Removing interface: batadv_slave_0
[   55.260433][   T12] batman_adv: batadv0: Interface deactivated: batadv_slave_1
[   55.260445][   T12] batman_adv: batadv0: Removing interface: batadv_slave_1
[   55.277158][   T12] veth1_macvtap: left promiscuous mode
[   55.277216][   T12] veth0_macvtap: left promiscuous mode
[   55.277298][   T12] veth1_vlan: left promiscuous mode
[   55.277352][   T12] veth0_vlan: left promiscuous mode
[   55.467919][   T12] team0 (unregistering): Port device team_slave_1 removed
[   55.483319][   T12] team0 (unregistering): Port device team_slave_0 removed
[   55.574371][ T5449] 8021q: adding VLAN 0 to HW filter on device eth2
[   55.834084][ T5449] 8021q: adding VLAN 0 to HW filter on device eth3
SYZFAIL: failed to recv rpc
fd=3 want=4 recv=0 n=0 (errno 9: Bad file descriptor)
execution of simple program fails: NOTFAIL: arptable checkpoint: getsockopt(ARPT_SO_GET_ENTRIES) failed. table=filter (errno 22: Invalid argument).
[   56.022767][ T5449] 8021q: adding VLAN 0 to HW filter on device eth4
[   56.418782][ T5449] 8021q: adding VLAN 0 to HW filter on device eth5


syzkaller build log:
go env (err=<nil>)
AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE='auto'
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/syzkaller/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/syzkaller/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build4086096228=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/syzkaller/jobs/linux/gopath/src/github.com/google/syzkaller/go.mod'
GOMODCACHE='/syzkaller/jobs/linux/gopath/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/syzkaller/jobs/linux/gopath'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/syzkaller/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26.0'
GOWORK=''
PKG_CONFIG='pkg-config'

git status (err=<nil>)
HEAD detached at 1e72964b011
nothing to commit, working tree clean


tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
Makefile:31: run command via tools/syz-env for best compatibility, see:
Makefile:32: https://github.com/google/syzkaller/blob/master/docs/contributing.md#using-syz-env
go list -f '{{.Stale}}' -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=1e72964b0111319984575e60f266d1fa0a98abb5 -X github.com/google/syzkaller/prog.gitRevisionDate=20260817-123423"  ./sys/syz-sysgen | grep -q false || go install -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=1e72964b0111319984575e60f266d1fa0a98abb5 -X github.com/google/syzkaller/prog.gitRevisionDate=20260817-123423"  ./sys/syz-sysgen
make .descriptions
tput: No value for $TERM and no -T specified
tput: No value for $TERM and no -T specified
Makefile:31: run command via tools/syz-env for best compatibility, see:
Makefile:32: https://github.com/google/syzkaller/blob/master/docs/contributing.md#using-syz-env
bin/syz-sysgen
touch .descriptions
GOOS=linux GOARCH=amd64 go build -ldflags="-s -w -X github.com/google/syzkaller/prog.GitRevision=1e72964b0111319984575e60f266d1fa0a98abb5 -X github.com/google/syzkaller/prog.gitRevisionDate=20260817-123423"  -o ./bin/linux_amd64/syz-execprog github.com/google/syzkaller/tools/syz-execprog
mkdir -p ./bin/linux_386
x86_64-linux-gnu-g++ -o ./bin/linux_386/syz-executor executor/executor.cc \
	-m32 -O2 -pthread -Wall -Werror -Wparentheses -Wunused-const-variable -Wframe-larger-than=16384 -Wno-stringop-overflow -Wno-array-bounds -Wno-format-overflow -Wno-unused-but-set-variable -Wno-unused-command-line-argument -static-pie -std=c++17 -I. -Iexecutor/_include   -DGOOS_linux=1 -DGOARCH_386=1 \
	-DHOSTGOOS_linux=1 -DGIT_REVISION=\"1e72964b0111319984575e60f266d1fa0a98abb5\"
/usr/bin/ld: /tmp/ccPHEVZY.o: in function `Connection::Connect(char const*, char const*)':
executor.cc:(.text._ZN10Connection7ConnectEPKcS1_[_ZN10Connection7ConnectEPKcS1_]+0x145): warning: Using 'gethostbyname' in statically linked applications requires at runtime the shared libraries from the glibc version used for linking
./tools/check-syzos.sh 2>/dev/null
[INFO] Unsupported architecture '386', skipping check.



Tested on:

commit:         72d3fcf8 Linux 7.3-rc5
git tree:       upstream
kernel config:  https://syzkaller.appspot.com/x/.config?x=a2a10c7c0ac9ebc2
dashboard link: https://syzkaller.appspot.com/bug?extid=57f410c9a4f8d7a441d6
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
userspace arch: i386
patch:          https://syzkaller.appspot.com/x/patch.diff?x=17c0f605580000