Subject: Re: [PATCH net v1] selftests: bonding: add test for stacked bond header_parse recursion

From: Jiayuan Chen <[email-masked]>
Date: Sun, 15 Mar 2026 18:40:51 +0800

On 3/15/26 1:38 AM, Jakub Kicinski wrote:
> On Sat, 14 Mar 2026 21:42:05 +0800 Jiayuan Chen wrote:
>> From: Jiayuan Chen <[email-masked]>
>>
>> Add a selftest to reproduce the infinite recursion in bond_header_parse()
>> when bonds are stacked (bond1 -> bond0 -> gre). When a packet is received
>> via AF_PACKET SOCK_DGRAM on the topmost bond, dev_parse_header() calls
>> bond_header_parse() which used skb->dev (always the topmost bond) to get
>> the bonding struct. This caused it to recurse back into itself
>> indefinitely, leading to stack overflow.
>>
>> Before Eric's fix [2], the test triggers:
>>
>>    ./bond-stacked-header-parse.sh
>>
>>    [  71.999481] BUG: MAX_LOCK_DEPTH too low!
>>   
[...]
>> +	ip link add name "$devbond0" type bond mode active-backup
>> +	check_err $? "could not create bond0"
>> +	ip link add name "$devbond1" type bond mode active-backup
>> +	check_err $? "could not create bond1"
>> +
>> +	ip link set "$devgre" master "$devbond0"
>> +	check_err $? "could not enslave $devgre to $devbond0"
>> +	ip link set "$devbond0" master "$devbond1"
>> +	check_err $? "could not enslave $devbond0 to $devbond1"
>> +
>> +	ip link set "$devgre" up
>> +	ip link set "$devbond0" up
>> +	ip link set "$devbond1" up
>> +
>> +	# Send a GRE-encapsulated packet to 10.0.0.1 while an AF_PACKET
>> +	# SOCK_DGRAM socket is listening on bond1. The receive path calls
>> +	# dev_parse_header() which invokes bond_header_parse(). With the
>> +	# bug, this recurses infinitely and causes a stack overflow.
>> +	#
>> +	# Use Python to:
>> +	# 1. Open AF_PACKET SOCK_DGRAM on bond1
>> +	# 2. Send a GRE packet to 10.0.0.1 via raw socket
>> +	# 3. Try to receive (triggers parse path)
>> +	python3 -c "
>> +import socket, struct, time
> is this AI-generated?
>
> You can add an extra script in TEST_FILES and just call it.
> No need for inline scripts..
>
>> +# AF_PACKET SOCK_DGRAM on bond1
>> +ETH_P_ALL = 0x0003
>> +pkt_fd = socket.socket(socket.AF_PACKET, socket.SOCK_DGRAM,
>> +                       socket.htons(ETH_P_ALL))
>> +pkt_fd.settimeout(2)
>> +pkt_fd.bind(('$devbond1', ETH_P_ALL))
>> +
>> +# Build GRE-encapsulated IP packet
>> +def build_ip_hdr(proto, saddr, daddr, payload_len):
>> +    ihl_ver = 0x45
>> +    total_len = 20 + payload_len
>> +    hdr = struct.pack('!BBHHHBBH4s4s',
>> +        ihl_ver, 0, total_len, 0, 0, 64, proto, 0,
>> +        socket.inet_aton(saddr), socket.inet_aton(daddr))
>> +    # compute checksum
>> +    words = struct.unpack('!10H', hdr)
>> +    s = sum(words)
>> +    while s >> 16:
>> +        s = (s & 0xffff) + (s >> 16)
>> +    chksum = ~s & 0xffff
>> +    hdr = hdr[:10] + struct.pack('!H', chksum) + hdr[12:]
>> +    return hdr
>> +
>> +inner = build_ip_hdr(17, '192.168.1.1', '192.168.1.2', 8) + b'\x00' * 8
>> +gre_hdr = struct.pack('!HH', 0, 0x0800)  # flags=0, proto=IP
>> +outer = build_ip_hdr(47, '10.0.0.2', '10.0.0.1', len(gre_hdr) + len(inner))
>> +pkt = outer + gre_hdr + inner
>> +
>> +raw_fd = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_RAW)
>> +raw_fd.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)
>> +raw_fd.sendto(pkt, ('10.0.0.1', 0))
>> +raw_fd.close()
>> +
>> +try:
>> +    pkt_fd.recv(2048)
>> +except socket.timeout:
>> +    pass
>> +pkt_fd.close()
>> +" 2>/dev/null
>> +
>> +	# If we get here without a kernel crash/hang, the test passed.
>> +	# Also check dmesg for signs of the recursion bug.
>> +	if dmesg | tail -20 | grep -q "BUG: MAX_LOCK_DEPTH\|stack-overflow\|stack overflow"; then
>> +		check_err 1 "kernel detected recursion in bond_header_parse"
>> +	fi
>> +
>> +	# Cleanup
>> +	ip link del "$devbond1" 2>/dev/null
>> +	ip link del "$devbond0" 2>/dev/null
>> +	ip link del "$devgre" 2>/dev/null
>> +	ip link del "$devdummy" 2>/dev/null
>> +
>> +	log_test "Stacked bond header_parse does not recurse"
>> +}
>> +
>> +require_command python3
> No need, we have pure python tests
>
>> +tests_run
>> +
>> +exit "$EXIT_STATUS"
Thanks Jakub,

All the feedback makes sense, will address them in v2.

Regarding the Python script, I originally had something simpler like:

# AF_PACKET SOCK_DGRAM on non-Ethernet device triggers dev_parse_header()
timeout 5 tcpdump -c 1 -i "$devbond1" >/dev/null 2>&1 &
sleep 1

# Send a GRE packet so it arrives via gre -> bond0 -> bond1
python3 -c "
from scapy.all import *
send(IP(src='10.0.0.2', dst='10.0.0.1')/GRE()/IP()/UDP(), verbose=0)
"

But I wasn't sure if scapy is an acceptable dependency for selftests,
and I also wasn't confident that tcpdump will always use AF_PACKET
SOCK_DGRAM internally. So I ended up writing the Python script to
handle both the SOCK_DGRAM listener and packet construction myself.

Is it fine to just inline a few lines like the scapy approach above,

or would you prefer keeping it as a separate script?