tracing: provide trace_seq_reserve()

trace_seq_reserve() allows a caller to reserve space in a trace_seq and
write directly into it. This makes it easier to export binary data to
userspace via the tracing interface, by simply filling in a struct.
Signed-off-by: NEduard - Gabriel Munteanu <eduard.munteanu@linux360.ro>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/trace/trace_output.c

@@ -167,6 +167,19 @@ int trace_seq_putmem_hex(struct trace_seq *s, void *mem, size_t len)
 	return trace_seq_putmem(s, hex, j);
 }
 
+void *trace_seq_reserve(struct trace_seq *s, size_t len)
+{
+	void *ret;
+
+	if (len > ((PAGE_SIZE - 1) - s->len))
+		return NULL;
+
+	ret = s->buffer + s->len;
+	s->len += len;
+
+	return ret;
+}
+
 int trace_seq_path(struct trace_seq *s, struct path *path)
 {
 	unsigned char *p;

kernel/trace/trace_output.h

@@ -33,6 +33,7 @@ int trace_seq_puts(struct trace_seq *s, const char *str);
 int trace_seq_putc(struct trace_seq *s, unsigned char c);
 int trace_seq_putmem(struct trace_seq *s, void *mem, size_t len);
 int trace_seq_putmem_hex(struct trace_seq *s, void *mem, size_t len);
+void *trace_seq_reserve(struct trace_seq *s, size_t len);
 int trace_seq_path(struct trace_seq *s, struct path *path);
 int seq_print_userip_objs(const struct userstack_entry *entry,
 			  struct trace_seq *s, unsigned long sym_flags);