CODEWITHSUNDEEP
clinuxnetwork-programmingpcap
AsamRegnat
AsamRegnat

Reputation: 91

pcap_next() returns address to inaccessible memory area

I'm programming an ARP-Sniffer and the next runtime error occures:

The pcap_next() function returns a pointer to an address space, which is not accessible causing the Segmentation Fault Error during the runtime only in some cases. Here is the code:

void function_arp(){
(...)

const unsigned char *frameRcv = NULL;
unsigned int byteNr = 0;
struct ether_arp *arpPack = NULL;

while (true) {
    // check on waiting time
    gettimeofday(&currentWaitTime, NULL);
    struct timeval diff = timediff(beginWaitTime, currentWaitTime);

    if (diff.tv_sec > 5) // wait for up to 5 seconds
    {
        fprintf(stderr, "Reading timed out\n");
        break;
    }

    // Receiving Frame
    byteNr = receiveRawFrame(handle, &frameRcv);
    if(frameRcv == 0x0) 
        continue;
    //when reading *frameRcv, the Segmentation fault occurs sometimes 
    fprintf(stdout, "%x - ", *frameRcv);  
    (...) 
    //The receiving of frames will be timed out after 4 sec
}

The code of the function receiveRawFrame():

unsigned int
receiveRawFrame(struct capture_info handle, const unsigned char** receivedFrame)
{
    *receivedFrame = NULL;
    // try reading frames
    const unsigned char* frame;
    struct pcap_pkthdr pcapinfo;
    frame = pcap_next(handle.pcapHandle, &pcapinfo);

    *receivedFrame = frame;
    return pcapinfo.caplen;
}

Some Notes: The Program will recieve as parameter the IP and send an ARP-Request for that IP expecting the reply afterwards. The Segmentation Fault occurs only when the IP is not in the local network. Normaly the program should time out in this case. For the same input (an IP outwards the local network) the Segmentation Fault occurs sometimes, sometimes not...

I suppose something unexpected occurs when calling the function pcap_next several times in a while loop.

The OS: Debian 3.2.57-3 x86_64 (64 Bit) Pcap version: 1.15

GDB is offering the next output:

Program received signal SIGSEGV, Segmentation fault.
0x0000000000401b29 in function_arp () at src/arp_impl.c:120
120                             fprintf(stdout, "%x - ", *frameRcv);

Upvotes: 0

Views: 545

Answers (1)

DrC
DrC

Reputation: 7698

Check the result from pcap_next. The frame and info should only be be accessed if the result is non-zero.

Upvotes: 2

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