pping: Add hint on XDP failure

Due to a kernel bug for XDP programs loaded via libxdp that use global
functions (see https://lore.kernel.org/bpf/[email protected]/t/),
XDP mode only works on relatively recent kernels where the bug is
patched (or kernels where the patch has been backported). As many
users may not have such a recent kernel they will only see a confusing
verifier error like the following:

Starting ePPing in standard mode tracking TCP on test123
libbpf: elf: skipping unrecognized data section(7) xdp_metadata
libbpf: elf: skipping unrecognized data section(7) xdp_metadata
libbpf: prog 'pping_xdp_ingress': BPF program load failed: Invalid argument
libbpf: prog 'pping_xdp_ingress': -- BEGIN PROG LOAD LOG --
Func#1 is safe for any args that match its prototype
Validating pping_xdp_ingress() func#0...
0: R1=ctx(id=0,off=0,imm=0) R10=fp0
; int pping_xdp_ingress(struct xdp_md *ctx)
0: (bf) r6 = r1                       ; R1=ctx(id=0,off=0,imm=0) R6_w=ctx(id=0,off=0,imm=0)
1: (b7) r7 = 0                        ; R7_w=invP0
; struct packet_info p_info = { 0 };
2: (7b) *(u64 *)(r10 -8) = r7         ; R7_w=invP0 R10=fp0 fp-8_w=00000000
3: (7b) *(u64 *)(r10 -16) = r7        ; R7_w=invP0 R10=fp0 fp-16_w=00000000
4: (7b) *(u64 *)(r10 -24) = r7        ; R7_w=invP0 R10=fp0 fp-24_w=00000000
5: (7b) *(u64 *)(r10 -32) = r7        ; R7_w=invP0 R10=fp0 fp-32_w=00000000
6: (7b) *(u64 *)(r10 -40) = r7        ; R7_w=invP0 R10=fp0 fp-40_w=00000000
7: (7b) *(u64 *)(r10 -48) = r7        ; R7_w=invP0 R10=fp0 fp-48_w=00000000
8: (7b) *(u64 *)(r10 -56) = r7        ; R7_w=invP0 R10=fp0 fp-56_w=00000000
9: (7b) *(u64 *)(r10 -64) = r7        ; R7_w=invP0 R10=fp0 fp-64_w=00000000
10: (7b) *(u64 *)(r10 -72) = r7       ; R7_w=invP0 R10=fp0 fp-72_w=00000000
11: (7b) *(u64 *)(r10 -80) = r7       ; R7_w=invP0 R10=fp0 fp-80_w=00000000
12: (7b) *(u64 *)(r10 -88) = r7       ; R7_w=invP0 R10=fp0 fp-88_w=00000000
13: (7b) *(u64 *)(r10 -96) = r7       ; R7_w=invP0 R10=fp0 fp-96_w=00000000
14: (7b) *(u64 *)(r10 -104) = r7      ; R7_w=invP0 R10=fp0 fp-104_w=00000000
15: (7b) *(u64 *)(r10 -112) = r7      ; R7_w=invP0 R10=fp0 fp-112_w=00000000
16: (7b) *(u64 *)(r10 -120) = r7      ; R7_w=invP0 R10=fp0 fp-120_w=00000000
17: (7b) *(u64 *)(r10 -128) = r7      ; R7_w=invP0 R10=fp0 fp-128_w=00000000
18: (bf) r2 = r10                     ; R2_w=fp0 R10=fp0
;
19: (07) r2 += -128                   ; R2=fp-128
; if (parse_packet_identifer_xdp(ctx, &p_info) < 0)
20: (85) call pc+13
R1 type=ctx expected=fp
Caller passes invalid args into func#1
processed 206542 insns (limit 1000000) max_states_per_insn 32 total_states 13238 peak_states 792 mark_read 40
-- END PROG LOAD LOG --
libbpf: failed to load program 'pping_xdp_ingress'
libbpf: failed to load object 'pping_kern.o'
Failed attaching ingress BPF program on interface test123: Invalid argument
Failed loading and attaching BPF programs in pping_kern.o

To help users that run into this issue, add a small hint suggesting to
upgrade the kernel or use the tc ingress mode instead in case
attaching the XDP program fails.

Signed-off-by: Simon Sundberg <[email protected]>

pping/pping.c

@@ -885,6 +885,9 @@ static int load_attach_bpfprogs(struct bpf_object **obj,
 		/* xdp_attach() loads 'obj' through libxdp */
 		err = xdp_attach(*obj, config->ingress_prog, config->ifindex,
 				 &config->xdp_prog);
+		if (err)
+			fprintf(stderr, "Failed attaching XDP program\n"
+				"Try updating kernel or use tc (--ingress-hook tc) instead\n");
 	} else {
 		err = bpf_object__load(*obj);
 		if (err) {