[PATCH v6 49/57] dyndbg: add structs _ddebug_hdr, _ddebug_site_hdr

[Jim Cromie]

Add new structs _ddebug_hdr and _ddebug_site_hdr, latter for symmetry.

The job of the struct _ddebug_hdr is to:

Contain an _uplink member, which points up to the _ddebug_info
record that keeps builtin and module's dyndbg state.  That record is
init'd (by dynamic_debug_init for builtins, or find_module_sections
for loadables).

Be declared in .gnu.linkonce.dyndbg section, which places it just
before the __dyndbg section; a fixed relative offset.

This allows _ddebug_map_site(desc) to cheaply compute
&descs[desc._index], &descs[0], then container_of() to get the header
and its ._uplink, to the _ddebug_info, which gives access to
->sites[desc->_map].

Structurally, the header has a single unnamed union (which exposes its
field names upward) containing an embedded struct _ddebug{,_site}
(this forces the size to be compatible with the array/section its
destined for), shared with our new struct _ddebug_info * _uplink
member, which is also inside an unnamed struct, to allow adding other
fields later to use the remaining space.

In dynamic_debug.c: ref the linker symbols for the headers, print them
to confirm expected linkages; things look proper.  Add a header ref to
struct _ddebug_info, to facilitate the validity checks.

Signed-off-by: Jim Cromie <jim.cromie at gmail.com>
---
 include/linux/dynamic_debug.h | 23 ++++++++++++++++++++++-
 lib/dynamic_debug.c           |  9 +++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index 268e903b7c4e..f23608c38a79 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -126,8 +126,29 @@ struct ddebug_class_map {
 #define NUM_TYPE_ARGS(eltype, ...)				\
         (sizeof((eltype[]){__VA_ARGS__}) / sizeof(eltype))
 
-/* encapsulate linker provided built-in (or module) dyndbg data */
+/* define header record, linker inserts it at descs[0] */
+struct _ddebug_hdr {
+	union {
+		struct _ddebug __;  /* force sizeof this */
+		struct {
+			struct _ddebug_info * _uplink;
+			/* space available */
+		};
+	};
+};
+/* here for symmetry, extra storage */
+struct _ddebug_site_hdr {
+	union {
+		struct _ddebug_site __;  /* force sizeof this */
+		struct {
+			struct _ddebug_info  * _uplink;
+		};
+	};
+};
+
+/* encapsulate linker provided built-in (or module) dyndbg vectors */
 struct _ddebug_info {
+	struct _ddebug_hdr *hdr;
 	struct _ddebug *descs;
 	struct _ddebug_site *sites;
 	struct ddebug_class_map *classes;
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index 65b0a1025ddf..91fe7fb5dda9 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -49,6 +49,9 @@ extern struct _ddebug_site __stop___dyndbg_sites[];
 extern struct ddebug_class_map __start___dyndbg_classes[];
 extern struct ddebug_class_map __stop___dyndbg_classes[];
 
+extern struct _ddebug_hdr __dyndbg_header[];
+extern struct _ddebug_site_hdr __dyndbg_site_header[];
+
 struct ddebug_table {
 	struct list_head link, maps;
 	const char *mod_name;
@@ -1522,6 +1525,7 @@ static int __init dynamic_debug_init(void)
 	char *cmdline;
 
 	struct _ddebug_info di = {
+		.hdr = __dyndbg_header,
 		.descs = __start___dyndbg,
 		.sites = __start___dyndbg_sites,
 		.classes = __start___dyndbg_classes,
@@ -1545,6 +1549,11 @@ static int __init dynamic_debug_init(void)
 		pr_err("unequal vectors: descs/sites %d/%d\n", di.num_descs, di.num_sites);
 		return 1;
 	}
+
+	/* these 2 print the same, until _TABLE is added */
+	v2pr_info("%px %px \n", __dyndbg_header, __dyndbg_site_header);
+	v2pr_info("%px %px \n", di.descs, di.sites);
+
 	iter = iter_mod_start = __start___dyndbg;
 	site = site_mod_start = __start___dyndbg_sites;
 	modname = site->_modname;
-- 
2.37.2