bpf: fix oob in btf_name_valid_section

Check the first char of the BTF DATASEC names.

Fixes: bd70a8f ("bpf: Allow all printable characters in BTF DATASEC names")
Reported-and-tested-by: syzbot+cc32304f6487ebff9b70@syzkaller.appspotmail.com
Signed-off-by: Edward Adam Davis <eadavis@qq.com>

Author: ea1davis

kernel/bpf/btf.c

@@ -816,6 +816,8 @@ static bool btf_name_valid_section(const struct btf *btf, u32 offset)
 	const char *src = btf_str_by_offset(btf, offset);
 	const char *src_limit;
 
+	if (!isprint(*src))
+		return false;
 	/* set a limit on identifier length */
 	src_limit = src + KSYM_NAME_LEN;
 	src++;