bpf: Fix "same insn cannot be used with different pointers" in proxy.h

When DEBUG is enabled, Clang 17 optimizes the final part of
assign_socket_tcp and assign_socket_udp by keeping only one instance of
the code and jumping to it from two places. It means that the same
struct bpf_sock *sk pointer can point to the return value of either
skc_lookup_tcp or sk_lookup_udp. Unfortunately, for the verifier these
pointer types are different (PTR_TO_SOCK_COMMON and PTR_TO_SOCKET), and
it doesn't allow such pattern with the following error:

; sk = sk_lookup_udp(ctx, tuple, len, BPF_F_CURRENT_NETNS, 0);
829: (bf) r1 = r6
830: (b4) w3 = 36
831: (b7) r4 = -1
832: (b7) r5 = 0
833: (85) call bpf_sk_lookup_udp#85
last_idx 833 first_idx 827
regs=8 stack=0 before 832: (b7) r5 = 0
regs=8 stack=0 before 831: (b7) r4 = -1
regs=8 stack=0 before 830: (b4) w3 = 36
834: (b4) w7 = -178
; if (!sk)
835: (15) if r0 == 0x0 goto pc+38
 R0_w=sock(id=0,ref_obj_id=29,off=0,imm=0) R6=ctx(id=0,off=0,imm=0) R7_w=inv4294967118 R8=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R9=inv(id=22,umax_value=65535,var_off=(0x0; 0xffff)) R10=fp0 fp-16=???mmmmm fp-24=mmmmmmmm fp-32=mmmmmmmm fp-40=mmmmmmmm fp-48=mmmmmmmm fp-72=????mmmm fp-80=mmmmmmmm fp-88=mmmmmmmm refs=29
;
836: (61) r1 = *(u32 *)(r6 +16)
837: (63) *(u32 *)(r10 -104) = r1
838: (61) r7 = *(u32 *)(r0 +4)
same insn cannot be used with different pointers
processed 1261 insns (limit 1000000) max_states_per_insn 4 total_states 68 peak_states 67 mark_read 19

Work around it by adding READ_ONCE to the problematic dereference.
Although it doesn't guarantee that the same problem won't happen, it
helps in this case.

Ref: #32169
Fixes: 673d481 ("images: Update LLVM to 17.0.6")
Reported-by: Marco Hofstetter <marco.hofstetter@isovalent.com>
Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
Co-developed-by: Marco Hofstetter <marco.hofstetter@isovalent.com>
Signed-off-by: Marco Hofstetter <marco.hofstetter@isovalent.com>

bpf/lib/proxy.h

@@ -28,7 +28,7 @@ assign_socket_tcp(struct __ctx_buff *ctx,
 	if (established && sk->state == BPF_TCP_LISTEN)
 		goto release;
 
-	dbg_ctx = sk->family << 16 | ctx->protocol;
+	dbg_ctx = READ_ONCE(sk)->family << 16 | ctx->protocol;
 	result = sk_assign(ctx, sk, 0);
 	cilium_dbg(ctx, DBG_SK_ASSIGN, -result, dbg_ctx);
 	if (result == 0)
@@ -54,7 +54,7 @@ assign_socket_udp(struct __ctx_buff *ctx,
 	if (!sk)
 		goto out;
 
-	dbg_ctx = sk->family << 16 | ctx->protocol;
+	dbg_ctx = READ_ONCE(sk)->family << 16 | ctx->protocol;
 	result = sk_assign(ctx, sk, 0);
 	cilium_dbg(ctx, DBG_SK_ASSIGN, -result, dbg_ctx);
 	if (result == 0)