tracing/probes: Fix to record 0-length data_loc in fetch_store_string*() if fails

Fix to record 0-length data to data_loc in fetch_store_string*() if it fails
to get the string data.
Currently those expect that the data_loc is updated by store_trace_args() if
it returns the error code. However, that does not work correctly if the
argument is an array of strings. In that case, store_trace_args() only clears
the first entry of the array (which may have no error) and leaves other
entries. So it should be cleared by fetch_store_string*() itself.
Also, 'dyndata' and 'maxlen' in store_trace_args() should be updated
only if it is used (ret > 0 and argument is a dynamic data.)

Link: https://lore.kernel.org/all/168908496683.123124.4761206188794205601.stgit@devnote2/

Fixes: 40b53b7 ("tracing: probeevent: Add array type support")
Cc: stable@vger.kernel.org
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Author: mhiramat

kernel/trace/trace_probe_kernel.h

@@ -37,6 +37,13 @@ fetch_store_strlen(unsigned long addr)
 	return (ret < 0) ? ret : len;
 }
 
+static nokprobe_inline void set_data_loc(int ret, void *dest, void *__dest, void *base)
+{
+	if (ret < 0)
+		ret = 0;
+	*(u32 *)dest = make_data_loc(ret, __dest - base);
+}
+
 /*
  * Fetch a null-terminated string from user. Caller MUST set *(u32 *)buf
  * with max length and relative data location.
@@ -55,8 +62,7 @@ fetch_store_string_user(unsigned long addr, void *dest, void *base)
 	__dest = get_loc_data(dest, base);
 
 	ret = strncpy_from_user_nofault(__dest, uaddr, maxlen);
-	if (ret >= 0)
-		*(u32 *)dest = make_data_loc(ret, __dest - base);
+	set_data_loc(ret, dest, __dest, base);
 
 	return ret;
 }
@@ -87,8 +93,7 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
 	 * probing.
 	 */
 	ret = strncpy_from_kernel_nofault(__dest, (void *)addr, maxlen);
-	if (ret >= 0)
-		*(u32 *)dest = make_data_loc(ret, __dest - base);
+	set_data_loc(ret, dest, __dest, base);
 
 	return ret;
 }

kernel/trace/trace_probe_tmpl.h

@@ -267,13 +267,9 @@ store_trace_args(void *data, struct trace_probe *tp, void *rec,
 		if (unlikely(arg->dynamic))
 			*dl = make_data_loc(maxlen, dyndata - base);
 		ret = process_fetch_insn(arg->code, rec, dl, base);
-		if (arg->dynamic) {
-			if (unlikely(ret < 0)) {
-				*dl = make_data_loc(0, dyndata - base);
-			} else {
-				dyndata += ret;
-				maxlen -= ret;
-			}
+		if (arg->dynamic && likely(ret > 0)) {
+			dyndata += ret;
+			maxlen -= ret;
 		}
 	}
 }

kernel/trace/trace_uprobe.c

@@ -170,7 +170,8 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
 			 */
 			ret++;
 		*(u32 *)dest = make_data_loc(ret, (void *)dst - base);
-	}
+	} else
+		*(u32 *)dest = make_data_loc(0, (void *)dst - base);
 
 	return ret;
 }