tracing/uprobes: Add @+file_offset fetch method

Enable to fetch data from a file offset.  Currently it only supports
fetching from same binary uprobe set.  It'll translate the file offset
to a proper virtual address in the process.

The syntax is "@+OFFSET" as it does similar to normal memory fetching
(@ADDR) which does no address translation.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: zhangwei(Jovi) <jovi.zhangwei@huawei.com>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index f94a569..ce0ed8a 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -239,6 +239,14 @@
 DEFINE_FETCH_symbol(string)
 DEFINE_FETCH_symbol(string_size)
 
+/* kprobes don't support file_offset fetch methods */
+#define fetch_file_offset_u8		NULL
+#define fetch_file_offset_u16		NULL
+#define fetch_file_offset_u32		NULL
+#define fetch_file_offset_u64		NULL
+#define fetch_file_offset_string	NULL
+#define fetch_file_offset_string_size	NULL
+
 /* Fetch type information table */
 const struct fetch_type kprobes_fetch_type_table[] = {
 	/* Special types */