GDB Examples
In this section we have a few useful gdb commands.
Starting GDB
Once at the gdb prompt, VPP can be started by running the following commands:
(gdb) run -c /etc/vpp/startup.conf
Starting program: /scratch/vpp-master/build-root/install-vpp_debug-native/vpp/bin/vpp -c /etc/vpp/startup.conf
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
vlib_plugin_early_init:361: plugin path /scratch/vpp-master/build-root/install-vpp_debug-native/vpp/lib/vpp_plugins:/scratch/vpp-master/build-root/install-vpp_debug-native/vpp/lib/vpp_plugins
....
Backtrace
If you encounter errors when running VPP, such as VPP terminating due to a segfault or abort signal, then you can run the VPP debug binary and then execute backtrace or bt.
(gdb) bt
#0 ip4_icmp_input (vm=0x7ffff7b89a40 <vlib_global_main>, node=0x7fffb6bb6900, frame=0x7fffb6725ac0) at /scratch/vpp-master/build-data/../src/vnet/ip/icmp4.c:187
#1 0x00007ffff78da4be in dispatch_node (vm=0x7ffff7b89a40 <vlib_global_main>, node=0x7fffb6bb 6900, type=VLIB_NODE_TYPE_INTERNAL, dispatch_state=VLIB_NODE_STATE_POLLING, frame=0x7fffb6725ac0, last_time_stamp=10581236529 65565) at /scratch/vpp-master/build-data/../src/vlib/main.c:988
#2 0x00007ffff78daa77 in dispatch_pending_node (vm=0x7ffff7b89a40 <vlib_global_main>, pending_frame_index=6, last_time_stamp=1058123652965565) at /scratch/vpp-master/build-data/../src/vlib/main.c:1138
....
Get to the GDB prompt
When VPP is running, you can get to the command prompt by pressing CTRL+C.
Breakpoints
When at the GDB prompt, set a breakpoint by running the commands below:
(gdb) break ip4_icmp_input
Breakpoint 4 at 0x7ffff6b9c00b: file /scratch/vpp-master/build-data/../src/vnet/ip/icmp4.c, line 142.
List the breakpoints already set:
(gdb) i b
Num Type Disp Enb Address What
1 breakpoint keep y 0x00007ffff6b9c00b in ip4_icmp_input at /scratch/vpp-master/build-data/../src/vnet/ip/icmp4.c:142
breakpoint already hit 3 times
2 breakpoint keep y 0x00007ffff6b9c00b in ip4_icmp_input at /scratch/vpp-master/build-data/../src/vnet/ip/icmp4.c:142
3 breakpoint keep y 0x00007ffff640f646 in tw_timer_expire_timers_internal_1t_3w_1024sl_ov
at /scratch/vpp-master/build-data/../src/vppinfra/tw_timer_template.c:775
Delete a breakpoint:
(gdb) del 2
(gdb) i b
Num Type Disp Enb Address What
1 breakpoint keep y 0x00007ffff6b9c00b in ip4_icmp_input at /scratch/vpp-master/build-data/../src/vnet/ip/icmp4.c:142
breakpoint already hit 3 times
3 breakpoint keep y 0x00007ffff640f646 in tw_timer_expire_timers_internal_1t_3w_1024sl_ov
at /scratch/vpp-master/build-data/../src/vppinfra/tw_timer_template.c:775
Step/Next/List
Step through the code using (s)tep into, (n)ext, and list some lines before and after where you are with list.
Thread 1 "vpp_main" hit Breakpoint 1, ip4_icmp_input (vm=0x7ffff7b89a40 <vlib_global_main>, node=0x7fffb6bb6900, frame=0x7fffb6709480)
at /scratch/jdenisco/vpp-master/build-data/../src/vnet/ip/icmp4.c:142
142 {
(gdb) n
143 icmp4_main_t *im = &icmp4_main;
(
(gdb) list
202 vlib_put_next_frame (vm, node, next, n_left_to_next);
203 }
204
205 return frame->n_vectors;
206 }
207
208 /* *INDENT-OFF* */
209 VLIB_REGISTER_NODE (ip4_icmp_input_node,static) = {
210 .function = ip4_icmp_input,
211 .name = "ip4-icmp-input",
Examining Data and packets
To look at data and packets use e(x)amine or (p)rint.
For example in this code look at the ip packet:
(gdb) p/x *ip0
$3 = {{ip_version_and_header_length = 0x45, tos = 0x0, length = 0x5400,
fragment_id = 0x7049, flags_and_fragment_offset = 0x40, ttl = 0x40, protocol = 0x1,
checksum = 0x2ddd, {{src_address = {data = {0xa, 0x0, 0x0, 0x2},
data_u32 = 0x200000a, as_u8 = {0xa, 0x0, 0x0, 0x2}, as_u16 = {0xa, 0x200},
as_u32 = 0x200000a}, dst_address = {data = {0xa, 0x0, 0x0, 0xa}, data_u32 = 0xa00000a,
as_u8 = {0xa, 0x0, 0x0, 0xa}, as_u16 = {0xa, 0xa00}, as_u32 = 0xa00000a}},
address_pair = {src = {data = {0xa, 0x0, 0x0, 0x2}, data_u32 = 0x200000a,
as_u8 = {0xa, 0x0, 0x0, 0x2}, as_u16 = {0xa, 0x200}, as_u32 = 0x200000a},
dst = {data = {0xa, 0x0, 0x0, 0xa}, data_u32 = 0xa00000a, as_u8 = {0xa, 0x0, 0x0, 0xa},
as_u16 = {0xa, 0xa00}, as_u32 = 0xa00000a}}}}, {checksum_data_64 =
{0x40704954000045, 0x200000a2ddd0140}, checksum_data_64_32 = {0xa00000a}},
{checksum_data_32 = {0x54000045, 0x407049, 0x2ddd0140, 0x200000a, 0xa00000a}}}
Then the icmp header
(gdb) p/x *icmp0
$4 = {type = 0x8, code = 0x0, checksum = 0xf148}
Then look at the actual bytes:
(gdb) x/50w ip0
0x7fde9953510e: 0x54000045 0x00407049 0x2ddd0140 0x0200000a
0x7fde9953511e: 0x0a00000a 0xf1480008 0x03000554 0x5b6b2e8a
0x7fde9953512e: 0x00000000 0x000ca99a 0x00000000 0x13121110
0x7fde9953513e: 0x17161514 0x1b1a1918 0x1f1e1d1c 0x23222120