bpf: Improve the info.func_info and info.func_info_rec_size behavior

1) When bpf_dump_raw_ok() == false and the kernel can provide >=1
   func_info to the userspace, the current behavior is setting
   the info.func_info_cnt to 0 instead of setting info.func_info
   to 0.

   It is different from the behavior in jited_func_lens/nr_jited_func_lens,
   jited_ksyms/nr_jited_ksyms...etc.

   This patch fixes it. (i.e. set func_info to 0 instead of
   func_info_cnt to 0 when bpf_dump_raw_ok() == false).

2) When the userspace passed in info.func_info_cnt == 0, the kernel
   will set the expected func_info size back to the
   info.func_info_rec_size.  It is a way for the userspace to learn
   the kernel expected func_info_rec_size introduced in
   commit 838e9690 ("bpf: Introduce bpf_func_info").

   An exception is the kernel expected size is not set when
   func_info is not available for a bpf_prog.  This makes the
   returned info.func_info_rec_size has different values
   depending on the returned value of info.func_info_cnt.

   This patch sets the kernel expected size to info.func_info_rec_size
   independent of the info.func_info_cnt.

3) The current logic only rejects invalid func_info_rec_size if
   func_info_cnt is non zero.  This patch also rejects invalid
   nonzero info.func_info_rec_size and not equal to the kernel
   expected size.

4) Set info.btf_id as long as prog->aux->btf != NULL.  That will
   setup the later copy_to_user() codes look the same as others
   which then easier to understand and maintain.

   prog->aux->btf is not NULL only if prog->aux->func_info_cnt > 0.

   Breaking up info.btf_id from prog->aux->func_info_cnt is needed
   for the later line info patch anyway.

   A similar change is made to bpf_get_prog_name().

Fixes: 838e9690 ("bpf: Introduce bpf_func_info")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/core.c

@@ -410,7 +410,7 @@ static void bpf_get_prog_name(const struct bpf_prog *prog, char *sym)
 	sym  = bin2hex(sym, prog->tag, sizeof(prog->tag));
 
 	/* prog->aux->name will be ignored if full btf name is available */
-	if (prog->aux->btf) {
+	if (prog->aux->func_info_cnt) {
 		type = btf_type_by_id(prog->aux->btf,
 				      prog->aux->func_info[prog->aux->func_idx].type_id);
 		func_name = btf_name_by_offset(prog->aux->btf, type->name_off);

kernel/bpf/syscall.c

@@ -2083,6 +2083,12 @@ static int bpf_prog_get_info_by_fd(struct bpf_prog *prog,
 				return -EFAULT;
 	}
 
+	if ((info.func_info_cnt || info.func_info_rec_size) &&
+	    info.func_info_rec_size != sizeof(struct bpf_func_info))
+		return -EINVAL;
+
+	info.func_info_rec_size = sizeof(struct bpf_func_info);
+
 	if (!capable(CAP_SYS_ADMIN)) {
 		info.jited_prog_len = 0;
 		info.xlated_prog_len = 0;
@@ -2226,35 +2232,23 @@ static int bpf_prog_get_info_by_fd(struct bpf_prog *prog,
 		}
 	}
 
-	if (prog->aux->btf) {
-		u32 krec_size = sizeof(struct bpf_func_info);
-		u32 ucnt, urec_size;
-
+	if (prog->aux->btf)
 		info.btf_id = btf_id(prog->aux->btf);
 
-		ucnt = info.func_info_cnt;
+	ulen = info.func_info_cnt;
 	info.func_info_cnt = prog->aux->func_info_cnt;
-		urec_size = info.func_info_rec_size;
-		info.func_info_rec_size = krec_size;
-		if (ucnt) {
-			/* expect passed-in urec_size is what the kernel expects */
-			if (urec_size != info.func_info_rec_size)
-				return -EINVAL;
-
+	if (info.func_info_cnt && ulen) {
 		if (bpf_dump_raw_ok()) {
 			char __user *user_finfo;
 
 			user_finfo = u64_to_user_ptr(info.func_info);
-				ucnt = min_t(u32, info.func_info_cnt, ucnt);
+			ulen = min_t(u32, info.func_info_cnt, ulen);
 			if (copy_to_user(user_finfo, prog->aux->func_info,
-						 krec_size * ucnt))
+					 info.func_info_rec_size * ulen))
 				return -EFAULT;
 		} else {
-				info.func_info_cnt = 0;
-			}
+			info.func_info = 0;
 		}
-	} else {
-		info.func_info_cnt = 0;
 	}
 
 done: