selftests/bpf: print correct offset for pseudo calls in disasm_insn()

Adjust disasm_helpers.c:disasm_insn() to account for the following
part of the verifier.c:jit_subprogs:

  for (i = 0, insn = prog->insnsi; i < prog->len; i++, insn++) {
        /* ... */
        if (!bpf_pseudo_call(insn))
                continue;
        insn->off = env->insn_aux_data[i].call_imm;
        subprog = find_subprog(env, i + insn->off + 1);
        insn->imm = subprog;
  }

Where verifier moves offset of the subprogram to the insn->off field.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240722233844.1406874-6-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

tools/testing/selftests/bpf/disasm_helpers.c

@@ -4,6 +4,7 @@
 #include "disasm.h"
 
 struct print_insn_context {
+	char scratch[16];
 	char *buf;
 	size_t sz;
 };
@@ -18,6 +19,22 @@ static void print_insn_cb(void *private_data, const char *fmt, ...)
 	va_end(args);
 }
 
+static const char *print_call_cb(void *private_data, const struct bpf_insn *insn)
+{
+	struct print_insn_context *ctx = private_data;
+
+	/* For pseudo calls verifier.c:jit_subprogs() hides original
+	 * imm to insn->off and changes insn->imm to be an index of
+	 * the subprog instead.
+	 */
+	if (insn->src_reg == BPF_PSEUDO_CALL) {
+		snprintf(ctx->scratch, sizeof(ctx->scratch), "%+d", insn->off);
+		return ctx->scratch;
+	}
+
+	return NULL;
+}
+
 struct bpf_insn *disasm_insn(struct bpf_insn *insn, char *buf, size_t buf_sz)
 {
 	struct print_insn_context ctx = {
@@ -26,6 +43,7 @@ struct bpf_insn *disasm_insn(struct bpf_insn *insn, char *buf, size_t buf_sz)
 	};
 	struct bpf_insn_cbs cbs = {
 		.cb_print	= print_insn_cb,
+		.cb_call	= print_call_cb,
 		.private_data	= &ctx,
 	};
 	char *tmp, *pfx_end, *sfx_start;