Vincent Li <mchun.li@xxxxxxxxx> writes:
> On Thu, 16 Jan 2020, Toke Høiland-Jørgensen wrote:
>
> Hi Toke,
>
>>
>> You could also try the xdp-loader in xdp-tools:
>> https://github.com/xdp-project/xdp-tools
>>
>> It's somewhat basic still, but should be able to at least load a basic
>> program - please file a bug report if it fails.
>
> I tried the xdp-tools xdp-loader, when the optlen is global variable, I
> got:
> # xdp-loader load enp3s0 tcp_option.o
> Couldn't load BPF program: Relocation failed
>
> if I move the optlen, i variable to local variable, I got:
>
> # xdp-loader load enp3s0 tcp_option.o
> Couldn't load eBPF object: Invalid argument(-22)
OK, I tried this, and there were a couple of issues:
- The xdp-loader didn't set the BPF program type to XDP, and since your
section name doesn't have an xdp_ prefix libbpf couldn't auto-detect
it. I've pushed a fix for this to the xdp-tools repo so the loader
will always set the program type to XDP now.
- There are a couple of bugs in your program:
First, when compiling with warnings turned on, I get this:
tcp_options.c:64:29: error: variable 'op' is uninitialized when used here [-Werror,-Wuninitialized]
if (op[i] == TCPOPT_EOL ) {
^~
tcp_options.c:43:23: note: initialize the variable 'op' to silence this warning
const __u8 *op;
^
= 0
after fixing that (adding this line after the optlen = assignment):
op = (const __u8 *)(tcphdr + 1);
the verifier then complains about out-of-bounds reading of the packet
data (pass -vv to xdp-loader to get the full debug output from libbpf).
You are not checking that the op pointer doesn't read out-of-bounds.
I fixed that by adding a couple of bounds checks inside the for loop.
The whole thing now looks like this:
optlen = tcphdr->doff*4 - sizeof(*tcphdr);
op = (const __u8 *)(tcphdr + 1);
for (i = 0; i < optlen; ) {
if ((void *)op + i + 1 > data_end)
return 0;
if (op[i] == TCPOPT_EOL ) {
char fmt[] = "XDP: tcp source : %d tcp option eol\n";
bpf_trace_printk(fmt, sizeof(fmt), (int)tcphdr->source);
return 1;
}
if (op[i] < 2)
i++;
else
i += ((void *)op + 2 < data_end && op[i+1]) ? : 1;
}
With this, I can successfully load the program using xdp-loader. Turning
the variables back into globals still doesn't work, but I think that
error message should be fairly obvious:
libbpf: invalid relo for 'op' in special section 0xfff2; forgot to initialize global var?..
-Toke
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