[PATCH v2 30/59] dyndbg: drop "protection" of class'd pr_debugs from legacy queries

[Jim Cromie]

Current classmap code protects class'd pr_debugs from unintended
changes by "legacy" unclassed queries:

  # this doesn't disable all of DRM_UT_* categories
  echo "-p" > /proc/dynamic_debug/control

  # name the class to change it - protective but tedious
  echo "class DRM_UT_CORE +p" > /proc/dynamic_debug/control

  # or do it the subsystem way
  echo 1 > /sys/module/drm/parameters/debug

This "name the class to change it" behavior gave a modicum of
protection to classmap users (ie DRM) so their debug settings aren't
trivially and unintentionally altered underneath them.

But this made the class keyword special in some sense; the other
keywords skip only on explicit mismatch, otherwize the code falls thru
to adjust the pr-debug site.

So Jason Baron didn't like this special case when I 1st proposed it;
I argued 2 points:
- "protection gives stable-debug, improving utility"
- __drm_debug is authoritative w/o dyndbg under it.

I thought I'd convinced him back then, (and the patchset got merged),
but he noted it again when he reviewed this series.  So this commit
names the "special case": ddebug_client_module_protects_classes(), and
reverts it to Jason's preference.

If a class mismatch is seen, code distinguishes whether the class was
explicitly given (and always skips/continue), or the DFLT was assumed
because no class was given.  Here we test
ddebug_client_module_protects_classes(), skip if so.

Later, if any user/module wants to protect its classes, we could add a
flag to ddebug_table, a means to set it from CLASSMAP_DEFINE, and
check it when applying a classless query/cmd.

CC: jbaron at akamai.com
Signed-off-by: Jim Cromie <jim.cromie at gmail.com>
---
 lib/dynamic_debug.c | 34 +++++++++++++++++++++++++---------
 1 file changed, 25 insertions(+), 9 deletions(-)

diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index c44502787c2b..13de0dd3a4ad 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -193,6 +193,17 @@ static int ddebug_find_valid_class(struct ddebug_table const *dt, const char *cl
 	return -ENOENT;
 }
 
+/*
+ * classmaps-v1 protected classes from changes by legacy commands
+ * (those selecting _DPRINTK_CLASS_DFLT by omission), v2 undoes that
+ * special treatment.  State so explicitly.  Later we could give
+ * modules the choice to protect their classes or to keep v2 behavior.
+ */
+static inline bool ddebug_client_module_protects_classes(const struct ddebug_table *dt)
+{
+	return false;
+}
+
 /*
  * Search the tables for _ddebug's which match the given `query' and
  * apply the `flags' and `mask' to them.  Returns number of matching
@@ -206,7 +217,7 @@ static int ddebug_change(const struct ddebug_query *query, struct flag_settings
 	unsigned int newflags;
 	unsigned int nfound = 0;
 	struct flagsbuf fbuf, nbuf;
-	int valid_class;
+	int slctd_class;
 
 	/* search for matching ddebugs */
 	mutex_lock(&ddebug_lock);
@@ -218,21 +229,26 @@ static int ddebug_change(const struct ddebug_query *query, struct flag_settings
 			continue;
 
 		if (query->class_string) {
-			valid_class = ddebug_find_valid_class(dt, query->class_string);
-			if (valid_class < 0)
+			slctd_class = ddebug_find_valid_class(dt, query->class_string);
+			if (slctd_class < 0)
+				/* skip/reject classes unknown by module */
 				continue;
 		} else {
-			/* constrain query, do not touch class'd callsites */
-			valid_class = _DPRINTK_CLASS_DFLT;
+			slctd_class = _DPRINTK_CLASS_DFLT;
 		}
 
 		for (i = 0; i < dt->info.descs.len; i++) {
 			struct _ddebug *dp = &dt->info.descs.start[i];
 
-			/* match site against query-class */
-			if (dp->class_id != valid_class)
-				continue;
-
+			if (dp->class_id != slctd_class) {
+				if (query->class_string)
+					/* site.class != given class */
+					continue;
+				/* legacy query, class'd site */
+				else if (ddebug_client_module_protects_classes(dt))
+					continue;
+				/* allow change on class'd pr_debug */
+			}
 			/* match against the source filename */
 			if (query->filename &&
 			    !match_wildcard(query->filename, dp->filename) &&
-- 
2.49.0