linux-kernel - [RFC PATCH v2 13/19] dyndbg: verify __dyndbg & __dyndbg

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Date:   Fri, 25 Dec 2020 13:19:38 -0700
From:   Jim Cromie <jim.cromie@...il.com>
To:     jbaron@...mai.com, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org
Cc:     Jim Cromie <jim.cromie@...il.com>
Subject: [RFC PATCH v2 13/19] dyndbg: verify __dyndbg & __dyndbg_callsite invariant

Prove that linker + DECLARE_DYNAMIC_DEBUG_METADATA reliably place the
2 related struct _ddebug* initializations into parallel/ordered slots
in the __dyndbg_* sections.

This is a step towards dropping the pointer between the 2 structs;
maybe the 2 vectors stay ordered, and we can deduce and use N.  Of
course this test won't survive, since it needs the pointer we seek to
drop, but its a start.

0- iterate over __dyndbg_callsite in parallel with __dyndbg
   rename var: s/iter_start/iter_mod_start/ for clarity, consistency.

I disregarded a checkpatch warning about externs in c-files, staying
consistent with long-standing code seemed better.

1- prove that iter->site == site_iter.
   DECLARE_DYNAMIC_DEBUG_METADATA + linker insure this now
   Maybe we can drop pointer, still get order.

WRT the debug-printing, its noisy, but only with verbose=3.
It warrants trimming later.

The offset grows smoothly, because it is N * sizeof(structs), which
differ.  It looks reliable.  Amend later to do math, converge on
truth.  If numbers are stable after stripping pointer, we have N.

   	       	       rec   	      ptr      	       mod-ptr N (void*)p
[    1.929072] dyndbg: 2828: ffffffff82b32f28 ffffffff82b32f10 1 24 40
[    1.929326] dyndbg: 2829: ffffffff82b32f40 ffffffff82b32f10 2 48 80
[    1.930209] dyndbg:   2 debug prints in module i386

We have N (col 4), and N * structsize (col 5).  I feel like it still
needs more staring at.

Signed-off-by: Jim Cromie <jim.cromie@...il.com>
---
 lib/dynamic_debug.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index 2d10fc1e16cd..c1a113460637 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -41,6 +41,8 @@
 
 extern struct _ddebug __start___dyndbg[];
 extern struct _ddebug __stop___dyndbg[];
+extern struct _ddebug_callsite __start___dyndbg_callsites[];
+extern struct _ddebug_callsite __stop___dyndbg_callsites[];
 
 struct ddebug_table {
 	struct list_head link;
@@ -119,6 +121,7 @@ do {								\
 
 #define vpr_info(fmt, ...)	vnpr_info(1, fmt, ##__VA_ARGS__)
 #define v2pr_info(fmt, ...)	vnpr_info(2, fmt, ##__VA_ARGS__)
+#define v3pr_info(fmt, ...)	vnpr_info(2, fmt, ##__VA_ARGS__)
 
 static void vpr_info_dq(const struct ddebug_query *query, const char *msg)
 {
@@ -1147,7 +1150,8 @@ static int __init dynamic_debug_init_control(void)
 
 static int __init dynamic_debug_init(void)
 {
-	struct _ddebug *iter, *iter_start;
+	struct _ddebug *iter, *iter_mod_start;
+	struct _ddebug_callsite *site, *site_mod_start;
 	const char *modname = NULL;
 	char *cmdline;
 	int ret = 0;
@@ -1162,23 +1166,33 @@ static int __init dynamic_debug_init(void)
 		ddebug_init_success = 1;
 		return 0;
 	}
-	iter = __start___dyndbg;
+
+	iter = iter_mod_start = __start___dyndbg;
+	site = site_mod_start = __start___dyndbg_callsites;
 	modname = iter->site->modname;
-	iter_start = iter;
-	for (; iter < __stop___dyndbg; iter++) {
+
+	for (; iter < __stop___dyndbg; iter++, site++) {
+
+		BUG_ON(site != iter->site);
+		v3pr_info("%u: %px %ld %ld %ld\n", entries, site,
+			  site - site_mod_start,
+			  ((void *)site - (void *)site_mod_start),
+			  ((void *)iter - (void *)iter_mod_start));
 		entries++;
+
 		if (strcmp(modname, iter->site->modname)) {
 			modct++;
-			ret = ddebug_add_module(iter_start, n, modname);
+			ret = ddebug_add_module(iter_mod_start, n, modname);
 			if (ret)
 				goto out_err;
 			n = 0;
 			modname = iter->site->modname;
-			iter_start = iter;
+			iter_mod_start = iter;
+			site_mod_start = site;
 		}
 		n++;
 	}
-	ret = ddebug_add_module(iter_start, n, modname);
+	ret = ddebug_add_module(iter_mod_start, n, modname);
 	if (ret)
 		goto out_err;
 
-- 
2.29.2