[dpdk-dev] Segmentation Fault on printf()

[sabu kurian]

Hello friends,

I'm trying to print the ether_type for a packet that I captured from a port
on my machine. Suppose 'm' holds the packet. 'm' is of type 'struct
rte_mbuf'. Intels API reference for DPDK says 'ether_type' is of uint16_t.
I used the following code to retrieve ether_type.

void * eth_type;
struct ether_hdr *eth;

eth = rte_pktmbuf_mtod(m, struct ether_hdr *);
eth_type = &eth->ether_type;

printf("\n Type is %" PRIu16 , *((uint16_t *)eth_type));

rte_pktmbuf_free(m);

I get a 'Segmentation fault' when the printf() statement gets executed.
Where am I going wrong ?

Thanks in advance

[Chris Wright]

Post by sabu kurian
Hello friends,
I'm trying to print the ether_type for a packet that I captured from a port
on my machine. Suppose 'm' holds the packet. 'm' is of type 'struct
rte_mbuf'. Intels API reference for DPDK says 'ether_type' is of uint16_t.
I used the following code to retrieve ether_type.
void * eth_type;
struct ether_hdr *eth;
eth = rte_pktmbuf_mtod(m, struct ether_hdr *);
eth_type = &eth->ether_type;
printf("\n Type is %" PRIu16 , *((uint16_t *)eth_type));

Looks ok, albeit slightly overly complicated.

struct ether_hdr *eth;
uint16_t eth_type;

eth = rte_pktmbuf_mtod(m, struct ether_hdr *);
eth_type = eth->ether_type;
printf("\n Type is %" PRIu16 , eth_type));

This would remove all the extra casting. Perhaps the mbuf is the problem
here. And if so, the above snippet would segfault on eth->ether_type
showing you that mbuf is invalid.

You could try to (above mtod):

rte_pktmbuf_dump(m, sizeof(struct ether_hdr));

as that will show key contents of mbuf and packet data (and do some
basic validation along the way).

thanks,
-chris

[sabu kurian]

Hello chris,

Thanks for your reply. I tried dumping the contents of mbuf as you have
suggested.
This is what I get:

dump mbuf at 0x0x6c7620, phys=7f34d91eba28, buf_len=59488
pkt_len=0, ol_flags=0, nb_segs=0, in_port=0

and soon after that I get a segmentation fault. Do you have any idea on
what could be causing it ?

Regards

Post by sabu kurian

Post by sabu kurian
Hello friends,
I'm trying to print the ether_type for a packet that I captured from a

port

Post by sabu kurian
on my machine. Suppose 'm' holds the packet. 'm' is of type 'struct
rte_mbuf'. Intels API reference for DPDK says 'ether_type' is of

uint16_t.

Post by sabu kurian
I used the following code to retrieve ether_type.
void * eth_type;
struct ether_hdr *eth;
eth = rte_pktmbuf_mtod(m, struct ether_hdr *);
eth_type = &eth->ether_type;
printf("\n Type is %" PRIu16 , *((uint16_t *)eth_type));

Looks ok, albeit slightly overly complicated.
struct ether_hdr *eth;
uint16_t eth_type;
eth = rte_pktmbuf_mtod(m, struct ether_hdr *);
eth_type = eth->ether_type;
printf("\n Type is %" PRIu16 , eth_type));
This would remove all the extra casting. Perhaps the mbuf is the problem
here. And if so, the above snippet would segfault on eth->ether_type
showing you that mbuf is invalid.
rte_pktmbuf_dump(m, sizeof(struct ether_hdr));
as that will show key contents of mbuf and packet data (and do some
basic validation along the way).
thanks,
-chris

[Daniel Kaminsky]

Sabu,

I think that in this case the packet length is zero which will result in
nothing from mtod.

Daniel

Post by sabu kurian
Hello chris,
Thanks for your reply. I tried dumping the contents of mbuf as you have
suggested.
dump mbuf at 0x0x6c7620, phys=7f34d91eba28, buf_len=59488
pkt_len=0, ol_flags=0, nb_segs=0, in_port=0
and soon after that I get a segmentation fault. Do you have any idea on
what could be causing it ?
Regards

Post by sabu kurian

Post by sabu kurian
Hello friends,
I'm trying to print the ether_type for a packet that I captured from a

port

Post by sabu kurian
on my machine. Suppose 'm' holds the packet. 'm' is of type 'struct
rte_mbuf'. Intels API reference for DPDK says 'ether_type' is of

uint16_t.

Post by sabu kurian
I used the following code to retrieve ether_type.
void * eth_type;
struct ether_hdr *eth;
eth = rte_pktmbuf_mtod(m, struct ether_hdr *);
eth_type = &eth->ether_type;
printf("\n Type is %" PRIu16 , *((uint16_t *)eth_type));

Looks ok, albeit slightly overly complicated.
struct ether_hdr *eth;
uint16_t eth_type;
eth = rte_pktmbuf_mtod(m, struct ether_hdr *);
eth_type = eth->ether_type;
printf("\n Type is %" PRIu16 , eth_type));
This would remove all the extra casting. Perhaps the mbuf is the problem
here. And if so, the above snippet would segfault on eth->ether_type
showing you that mbuf is invalid.
rte_pktmbuf_dump(m, sizeof(struct ether_hdr));
as that will show key contents of mbuf and packet data (and do some
basic validation along the way).
thanks,
-chris