BPF_SNPRINTF: R3 does not point to a readonly map

[huchen2021]

When I use BPF_SNPRINTF on my xdp.c, it reports the following error. If I don’t use BPF_SNPRINTF, the error is gone. Any hints will be appreaciated:

go run -exec sudo ./pkg ens160
2022/06/17 05:04:56 loading objects: field XdpProgFunc: program xdp_prog_func: load program: permission denied: ; int xdp_prog_func(struct xdp_md *ctx)
0: (b7) r0 = 0
; void *data_end = (void *)(long)ctx->data_end;
1: (61) r9 = *(u32 *)(r1 +4)
; void *data = (void *)(long)ctx->data;
2: (61) r6 = *(u32 *)(r1 +0)
; if (data + sizeof(struct ethhdr) > data_end)
3: (bf) r1 = r6
4: (07) r1 += 14
; if (data + sizeof(struct ethhdr) > data_end)
5: (2d) if r1 > r9 goto pc+693
 R0_w=inv0 R1_w=pkt(id=0,off=14,r=14,imm=0) R6_w=pkt(id=0,off=0,r=14,imm=0) R9_w=pkt_end(id=0,off=0,imm=0) R10=fp0
; if (ethh->h_proto != bpf_htons(ETH_P_IP))
6: (71) r1 = *(u8 *)(r6 +12)
7: (71) r2 = *(u8 *)(r6 +13)
8: (67) r2 <<= 8
9: (4f) r2 |= r1
10: (b7) r0 = 2
; if (ethh->h_proto != bpf_htons(ETH_P_IP))
11: (55) if r2 != 0x8 goto pc+687
 R0_w=inv2 R1_w=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R2_w=inv8 R6_w=pkt(id=0,off=0,r=14,imm=0) R9_w=pkt_end(id=0,off=0,imm=0) R10=fp0
; if (data + sizeof(struct ethhdr) + sizeof(struct iphdr) > data_end)
12: (bf) r7 = r6
13: (07) r7 += 34
14: (b7) r0 = 0
; if (data + sizeof(struct ethhdr) + sizeof(struct iphdr) > data_end)
15: (2d) if r7 > r9 goto pc+683
 R0=inv0 R1=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R2=inv8 R6=pkt(id=0,off=0,r=34,imm=0) R7=pkt(id=0,off=34,r=34,imm=0) R9=pkt_end(id=0,off=0,imm=0) R10=fp0
; if (iph->protocol != IPPROTO_TCP)
16: (71) r1 = *(u8 *)(r6 +23)
17: (b7) r0 = 2
; if (iph->protocol != IPPROTO_TCP)
18: (55) if r1 != 0x6 goto pc+680
 R0_w=inv2 R1_w=inv6 R2=inv8 R6=pkt(id=0,off=0,r=34,imm=0) R7=pkt(id=0,off=34,r=34,imm=0) R9=pkt_end(id=0,off=0,imm=0) R10=fp0
; if (data + sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct tcphdr) > data_end)
19: (bf) r1 = r6
20: (07) r1 += 54
21: (b7) r0 = 0
; if (data + sizeof(struct ethhdr) + sizeof(struct iphdr) + sizeof(struct tcphdr) > data_end)
22: (2d) if r1 > r9 goto pc+676
 R0_w=inv0 R1_w=pkt(id=0,off=54,r=54,imm=0) R2=inv8 R6=pkt(id=0,off=0,r=54,imm=0) R7=pkt(id=0,off=34,r=54,imm=0) R9=pkt_end(id=0,off=0,imm=0) R10=fp0
23: (b7) r1 = 0
; char ip_str[18] = {};
24: (6b) *(u16 *)(r10 -8) = r1
last_idx 24 first_idx 15
regs=2 stack=0 before 23: (b7) r1 = 0
25: (7b) *(u64 *)(r10 -16) = r1
26: (7b) *(u64 *)(r10 -24) = r1
27: (bf) r1 = r10
28: (07) r1 += -24
29: (bf) r4 = r10
30: (07) r4 += -40
; BPF_SNPRINTF(ip_str, sizeof(ip_str), "hello");
31: (b7) r2 = 18
32: (18) r3 = 0xffff8a264ae74923
34: (b7) r5 = 0
35: (85) call bpf_snprintf#165
last_idx 35 first_idx 15
regs=4 stack=0 before 34: (b7) r5 = 0
regs=4 stack=0 before 32: (18) r3 = 0xffff8a264ae74923
regs=4 stack=0 before 31: (b7) r2 = 18
R3 does not point to a readonly map'
processed 35 insns (limit 1000000) max_states_per_insn 0 total_states 1 peak_states 1 mark_read 1
exit status 1

[lmb]

From @dylandreimerink:

You are using a string literal directly in snprintf BPF_SNPRINTF(ip_str, sizeof(ip_str), "hello"). You must define it as a static const variable, which will cause the compiler to put it in the global data section(basically a read-only array map, hence the error).
The easiest way to fix it is to use the libbpf helper macros which do this for you so you can use the same syntax without thinking about it: https://github.com/libbpf/libbpf/blob/e5ff285a445af864af7f38e0b5c6904b099c9b7b/src/bpf_helpers.h#L231

[orishuss]

I encounter the same error, and using libbpf's helper macros does not help.
load program: permission denied: regs=4 stack=0 before 20: (b7) r2 = 17: R3 does not point to a readonly map' (41 line(s) omitted)
Does the compiler sometime not create a readonly map, even though the fmt variable is static const?
I'm using the BPF_SNPRINTF macro. uname -r shows 5.15.0-1011-aws. Also using clang-14.