tracing: probeevent: Correctly update remaining space in dynamic area

Commit 9178412 ("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/[email protected]

Cc: Ingo Molnar <[email protected]>
Cc: [email protected]
Fixes: 9178412 ("tracing: probeevent: Return consumed bytes of dynamic area")
Acked-by: Masami Hiramatsu <[email protected]>
Signed-off-by: Andreas Ziegler <[email protected]>
Signed-off-by: Steven Rostedt (VMware) <[email protected]>

Author: rupran

kernel/trace/trace_probe_tmpl.h

@@ -180,10 +180,12 @@ store_trace_args(void *data, struct trace_probe *tp, struct pt_regs *regs,
 		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;
+		}
 	}
 }