linux-kernel - [RFC PATCH v5 27/28] dyndbg: unionize _ddebug*_headers with struct

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Date:   Tue, 11 May 2021 12:50:56 -0600
From:   Jim Cromie <jim.cromie@...il.com>
To:     Jason Baron <jbaron@...mai.com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, Jim Cromie <jim.cromie@...il.com>
Subject: [RFC PATCH v5 27/28] dyndbg: unionize _ddebug*_headers with struct _ddebug*

Up until now, to create our header record (pair), we reused struct
_ddebug(|_sites), initializing the pair with "impossible" field values
(for a pr_debug() site), which makes it recognizable as a header not a
pr-debug callsite.

Now we want to specialize the header; differentiate it from its model,
then we can drop _ddebug.site, and use header.site indirectly.

This commit has several elements combining:

Update DEFINE_DYNAMIC_DEBUG_TABLE by replacing "struct _ddebug*" with
"union _ddebug*_header".  This change places the new object types into
our 2 elf sections.

our 2 new union types have 2 jobs:

a.fit properly in the section/array.  We do this by putting the struct
  inside the union, making the unions the same size as their contained
  structs.  This was sufficient to fix a data-misalignment crash on
  2nd loop (ie 1st record after header) in _init.

b.unions need the same field defns as their models, as refd in:
  DEFINE_DYNAMIC_DEBUG_TABLE() - inits
  is_dyndbg_header_pair(i,s)   - validates that init
  use anonymous struct.

RFC: My "anonymous union/struct fu" feels incomplete.  The struct in b
must contain all the fields of its outer union's own contained struct,
maybe even in the same order (tho not obvious yet).

Signed-off-by: Jim Cromie <jim.cromie@...il.com>
---
 include/linux/dynamic_debug.h | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index 46901b5348ee..9ca413985fb2 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -36,6 +36,16 @@ struct _ddebug_site {
 	const char *function;
 } __aligned(8);

+/* verbatim copy of struct _ddebug_site. nothing to save here. */
+union _ddebug_site_header {
+	struct _ddebug_site __;		/* inherit footprint */
+	struct {			/* header validation */
+		const char *modname;
+		const char *filename;
+		const char *function;
+	};
+};
+
 struct _ddebug {
 	SITE_CHK_(struct _ddebug_site *site;)
 	/* format is always needed, lineno shares word with flags */
@@ -74,6 +84,21 @@ struct _ddebug {
 #endif
 } __aligned(8);

+/*
+ * specialized version of struct _ddebug.  The only field we NEED is
+ * .site, since keeping it here allows dropping from struct _ddebug
+ * itself.  format, lineno are used to validate header records.
+ */
+union _ddebug_header {
+	struct _ddebug __;		/* inherit footprint */
+	struct {			/* header validation */
+		union _ddebug_site_header *site;
+		const char *format;
+		const unsigned lineno:18;
+		unsigned _index:14;
+		unsigned int flags:8;
+	};
+};

 #if defined(CONFIG_DYNAMIC_DEBUG_CORE)

@@ -136,10 +161,10 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
  */
 #define DEFINE_DYNAMIC_DEBUG_TABLE()				\
 	/* forward decl, allowing loopback pointer */		\
-	__weak struct _ddebug __used __aligned(8)		\
+	__weak union _ddebug_header __used __aligned(8)		\
 		__section(".gnu.linkonce.dyndbg")		\
 		_LINKONCE_dyndbg_header;			\
-	__weak struct _ddebug_site __used __aligned(8)		\
+	__weak union _ddebug_site_header __used __aligned(8)	\
 		__section(".gnu.linkonce.dyndbg_site")		\
 	_LINKONCE_dyndbg_site_header = {			\
 		.modname = KBUILD_MODNAME,			\
@@ -147,7 +172,7 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
 		/* forced pointer loopback, for distinction */	\
 		.filename = (void *) &_LINKONCE_dyndbg_header	\
 	};							\
-	__weak struct _ddebug __used __aligned(8)		\
+	__weak union _ddebug_header __used __aligned(8)		\
 		__section(".gnu.linkonce.dyndbg")		\
 	_LINKONCE_dyndbg_header = {				\
 		SITE_INIT_(_LINKONCE_dyndbg_site_header)	\
-- 
2.31.1