bpf: Set the number of exception entries properly for subprograms

Currently, if a bpf program has more than one subprograms, each program will be
jitted separately. For programs with bpf-to-bpf calls the
prog->aux->num_exentries is not setup properly. For example, with
bpf_iter_netlink.c modified to force one function to be not inlined and with
CONFIG_BPF_JIT_ALWAYS_ON the following error is seen:
   $ ./test_progs -n 3/3
   ...
   libbpf: failed to load program 'iter/netlink'
   libbpf: failed to load object 'bpf_iter_netlink'
   libbpf: failed to load BPF skeleton 'bpf_iter_netlink': -4007
   test_netlink:FAIL:bpf_iter_netlink__open_and_load skeleton open_and_load failed
   #3/3 netlink:FAIL
The dmesg shows the following errors:
   ex gen bug
which is triggered by the following code in arch/x86/net/bpf_jit_comp.c:
   if (excnt >= bpf_prog->aux->num_exentries) {
     pr_err("ex gen bug\n");
     return -EFAULT;
   }

This patch fixes the issue by computing proper num_exentries for each
subprogram before calling JIT.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: yonghong-song

kernel/bpf/verifier.c

@@ -9801,7 +9801,7 @@ static int jit_subprogs(struct bpf_verifier_env *env)
 	int i, j, subprog_start, subprog_end = 0, len, subprog;
 	struct bpf_insn *insn;
 	void *old_bpf_func;
-	int err;
+	int err, num_exentries;
 
 	if (env->subprog_cnt <= 1)
 		return 0;
@@ -9876,6 +9876,14 @@ static int jit_subprogs(struct bpf_verifier_env *env)
 		func[i]->aux->nr_linfo = prog->aux->nr_linfo;
 		func[i]->aux->jited_linfo = prog->aux->jited_linfo;
 		func[i]->aux->linfo_idx = env->subprog_info[i].linfo_idx;
+		num_exentries = 0;
+		insn = func[i]->insnsi;
+		for (j = 0; j < func[i]->len; j++, insn++) {
+			if (BPF_CLASS(insn->code) == BPF_LDX &&
+			    BPF_MODE(insn->code) == BPF_PROBE_MEM)
+				num_exentries++;
+		}
+		func[i]->aux->num_exentries = num_exentries;
 		func[i] = bpf_int_jit_compile(func[i]);
 		if (!func[i]->jited) {
 			err = -ENOTSUPP;