[RFC PATCH v6 28/34] dyndbg: unionize _ddebug*_headers with struct _ddebug*

Jim Cromie
2021-05-29T20:03:44Z

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
is really just setup, next patch finishes conversion.

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.

TBD: u8[sizeof(struct _ddebug)] would also fit properly.

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@gmail.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 7acd4c88411d..c9c568a698a8 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 = &_LINKONCE_dyndbg_site_header, \
--
2.31.1