[S390] s390dbf: Add description for usage of "%s" in sprintf events

Using "%s" in sprintf event functions is dangerous. This patch adds a short
description for this issue to the s390 debug feature documentation.
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

Documentation/s390/s390dbf.txt

@@ -495,6 +495,13 @@ and for each vararg a long value. So e.g. for a debug entry with a format
 string plus two varargs one would need to allocate a (3 * sizeof(long)) 
 byte data area in the debug_register() function.
 
+IMPORTANT: Using "%s" in sprintf event functions is dangerous. You can only
+use "%s" in the sprintf event functions, if the memory for the passed string is
+available as long as the debug feature exists. The reason behind this is that
+due to performance considerations only a pointer to the string is stored in
+the debug feature. If you log a string that is freed afterwards, you will get
+an OOPS when inspecting the debug feature, because then the debug feature will
+access the already freed memory.
 
 NOTE: If using the sprintf view do NOT use other event/exception functions
 than the sprintf-event and -exception functions.

arch/s390/include/asm/debug.h

@@ -167,6 +167,10 @@ debug_text_event(debug_info_t* id, int level, const char* txt)
         return debug_event_common(id,level,txt,strlen(txt));
 }
 
+/*
+ * IMPORTANT: Use "%s" in sprintf format strings with care! Only pointers are
+ * stored in the s390dbf. See Documentation/s390/s390dbf.txt for more details!
+ */
 extern debug_entry_t *
 debug_sprintf_event(debug_info_t* id,int level,char *string,...)
 	__attribute__ ((format(printf, 3, 4)));
@@ -206,7 +210,10 @@ debug_text_exception(debug_info_t* id, int level, const char* txt)
         return debug_exception_common(id,level,txt,strlen(txt));
 }
 
-
+/*
+ * IMPORTANT: Use "%s" in sprintf format strings with care! Only pointers are
+ * stored in the s390dbf. See Documentation/s390/s390dbf.txt for more details!
+ */
 extern debug_entry_t *
 debug_sprintf_exception(debug_info_t* id,int level,char *string,...)
 	__attribute__ ((format(printf, 3, 4)));