rxrpc: Fix oops in tracepoint

If the rxrpc_eproto tracepoint is enabled, an oops will be cause by the
trace line that rxrpc_extract_header() tries to emit when a protocol error
occurs (typically because the packet is short) because the call argument is
NULL.

Fix this by using ?: to assume 0 as the debug_id if call is NULL.

This can then be induced by:

echo -e '\0\0\0\0\0\0\0\0' | ncat -4u --send-only <addr> 20001

where addr has the following program running on it:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <linux/rxrpc.h>
int main(void)
{
struct sockaddr_rxrpc srx;
int fd;
memset(&srx, 0, sizeof(srx));
srx.srx_family = AF_RXRPC;
srx.srx_service = 0;
srx.transport_type = AF_INET;
srx.transport_len = sizeof(srx.transport.sin);
srx.transport.sin.sin_family = AF_INET;
srx.transport.sin.sin_port = htons(0x4e21);
fd = socket(AF_RXRPC, SOCK_DGRAM, AF_INET6);
bind(fd, (struct sockaddr *)&srx, sizeof(srx));
sleep(20);
return 0;
}

It results in the following oops.

BUG: kernel NULL pointer dereference, address: 0000000000000340
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page
...
RIP: 0010:trace_event_raw_event_rxrpc_rx_eproto+0x47/0xac
...
Call Trace:
<IRQ>
rxrpc_extract_header+0x86/0x171
? rcu_read_lock_sched_held+0x5d/0x63
? rxrpc_new_skb+0xd4/0x109
rxrpc_input_packet+0xef/0x14fc
? rxrpc_input_data+0x986/0x986
udp_queue_rcv_one_skb+0xbf/0x3d0
udp_unicast_rcv_skb.isra.8+0x64/0x71
ip_protocol_deliver_rcu+0xe4/0x1b4
ip_local_deliver+0xf0/0x154
__netif_receive_skb_one_core+0x50/0x6c
netif_receive_skb_internal+0x26b/0x2e9
napi_gro_receive+0xf8/0x1da
rtl8169_poll+0x303/0x4c4
net_rx_action+0x10e/0x333
__do_softirq+0x1a5/0x38f
irq_exit+0x54/0xc4
do_IRQ+0xda/0xf8
common_interrupt+0xf/0xf
</IRQ>
...
? cpuidle_enter_state+0x23c/0x34d
cpuidle_enter+0x2a/0x36
do_idle+0x163/0x1ea
cpu_startup_entry+0x1d/0x1f
start_secondary+0x157/0x172
secondary_startup_64+0xa4/0xb0

Fixes: a25e21f0bcd2 ("rxrpc, afs: Use debug_ids rather than pointers in traces")
Signed-off-by: David Howells <[e-mail]>
Reviewed-by: Marc Dionne <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/trace/events/rxrpc.h

···
 			     ),
 
 	    TP_fast_assign(
-		    __entry->call = call->debug_id;
+		    __entry->call = call ? call->debug_id : 0;
 		    __entry->serial = serial;
 		    __entry->why = why;
 			   ),