dyndbg: allow anchored match on format query term

This should work:

  echo module=amd* format=^[IF_TRACE]: +p  >/proc/dynamic_debug/control

consider drivers/gpu/drm/amd/display/include/logger_types.h:
It has 11 defines like:

  #define DC_LOG_IF_TRACE(...) pr_debug("[IF_TRACE]:"__VA_ARGS__)

These defines are used 804 times at recent count; they are a good use
case to evaluate existing format-message based classifications of
*pr_debug*.  Those macros prefix the supplied format with a fixed
string, I'd expect most existing message classification schemes to do
something similar.

Hence we want to be able to anchor our match to the beginning of the
format string, allowing easy construction of clear and precise
queries, leveraging the existing classification scheme to enable and
disable those callsites.

Note that unlike other search terms, formats are implicitly floating
substring matches, without the need for explicit wildcards.

This makes no attempt at wider regex features, just the one we need.

TLDR: Using the anchor also means the []s are less helpful for
disamiguating the prefix from a random in-message occurrence, allowing
shorter prefixes.

Acked-by: <jbaron@akamai.com>
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Link: https://lore.kernel.org/r/20200719231058.1586423-17-jim.cromie@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/dynamic_debug.c

@@ -181,9 +181,16 @@ static int ddebug_change(const struct ddebug_query *query,
 				continue;
 
 			/* match against the format */
-			if (query->format &&
-			    !strstr(dp->format, query->format))
-				continue;
+			if (query->format) {
+				if (*query->format == '^') {
+					char *p;
+					/* anchored search. match must be at beginning */
+					p = strstr(dp->format, query->format+1);
+					if (p != dp->format)
+						continue;
+				} else if (!strstr(dp->format, query->format))
+					continue;
+			}
 
 			/* match against the line number range */
 			if (query->first_lineno &&