[PATCH 4.20 77/92] tracing: probeevent: Correctly update remaining space in dynamic area

[Greg Kroah-Hartman]

4.20-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Andreas Ziegler <andreas.ziegler@xxxxxx>

commit f6675872db57305fa957021efc788f9983ed3b67 upstream.

Commit 9178412ddf5a ("tracing: probeevent: Return consumed
bytes of dynamic area") improved the string fetching
mechanism by returning the number of required bytes after
copying the argument to the dynamic area. However, this
return value is now only used to increment the pointer
inside the dynamic area but misses updating the 'maxlen'
variable which indicates the remaining space in the dynamic
area.

This means that fetch_store_string() always reads the *total*
size of the dynamic area from the data_loc pointer instead of
the *remaining* size (and passes it along to
strncpy_from_{user,unsafe}) even if we're already about to
copy data into the middle of the dynamic area.

Link: http://lkml.kernel.org/r/20190206190013.16405-1-andreas.ziegler@xxxxxx

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 9178412ddf5a ("tracing: probeevent: Return consumed bytes of dynamic area")
Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Signed-off-by: Andreas Ziegler <andreas.ziegler@xxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/trace/trace_probe_tmpl.h |    6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

--- a/kernel/trace/trace_probe_tmpl.h
+++ b/kernel/trace/trace_probe_tmpl.h
@@ -180,10 +180,12 @@ store_trace_args(void *data, struct trac
 		if (unlikely(arg->dynamic))
 			*dl = make_data_loc(maxlen, dyndata - base);
 		ret = process_fetch_insn(arg->code, regs, dl, base);
-		if (unlikely(ret < 0 && arg->dynamic))
+		if (unlikely(ret < 0 && arg->dynamic)) {
 			*dl = make_data_loc(0, dyndata - base);
-		else
+		} else {
 			dyndata += ret;
+			maxlen -= ret;
+		}
 	}
 }