[RFC PATCH v6 19/34] dyndbg: RFC - DEFINE_DYNAMIC_DEBUG_TABLE

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

DEFINE_DYNAMIC_DEBUG_TABLE is based on DEFINE_DYNAMIC_DEBUG_METADATA.
Like its model, it creates/allocates a pair of structs: _ddebug &
_ddebug_site. It inits them distinctively, so is_dyndbg_header_pair()
macro can verify them.

Its purpose is to reserve a single pair of header records in the front
of the "__dyndbgs" & "__dyndbg_sites" sections. This has several parts;

- the pair of structs are defined in 2 .gnu.linkonce.dyndbg* sections
corresponding to the 2 dyndbg* sections populated by _METADATA

- vmlinux.lds.h places these 2 new sections into the dyndbg* sections,
right after the start___dyndbg*s, before existing contents.

- the pair has "__used __weak" to qualm compiler concerns.
explicit externs were problematic with initialization, static decls too.

With this, the header record is now really __dyndbg[0].

Notes:

DYNAMIC_DEBUG is a 'transparent' facility, in that pr_debug() users
get extra features without additional api. Because of this,
DEFINE_DYNAMIC_DEBUG_TABLE is invoked by dynamic_debug.h on behalf of
all its (indirect) users, including printk.h.

IOW this has wide effects; it resulted in multiple redundant
definitions of header records. By hunch then trial and error, using
.gnu.linkonce. sections and "__used __weak " modifiers resolved the
problems. RFC.

In vmlinux-lds.h, I added 2 more KEEPs to place the .gnu.linkonce.*
headers at the front of their respective __dyndbg* sections. I moved
the __dyndbg lines into a new macro, which maybe should be done as a
separate commit, or in the earlier commit that touched it.

The headers are really just struct _ddebug*s, they are initialized
distinctively so that they're recogizable by code, using macro
is_dyndbg_header_pair(dbg, dbgsite).

DECLARE_DYNAMIC_DEBUG_TABLE() initializes the header record pairs with
values which are "impossible" for pr_debug()s: (and therefor distinctive)

- lineno == 0
- site == iter->site
- modname == function not possible in proper linkage
- modname == format not possible in normal linkage
- filename = (void*) iter forced loopback

Next:

Get __dyndbg[0] from any *dp within __dyndbg[N]. Then with
__dyndbg[0].site, we can get __dyndbg_sites[N]. This is a little
slower than a direct pointer, but this is an unlikely debug path, so
this 'up-N-over-down-N' access is ok.

Eventually we can adapt the header (as a new struct/union) to keep its
pointer to the __dyndbg_sites[] section/block, while allowing us to
drop it from struct _ddebug, regaining memory parity with master. But
for now, we keep .site, and will soon use it to validate the
'up-N-over-down-N' computation.

For clarity, N is _ddebug._index. For both builtins & loadable
modules, it is init'd to remember the fixed offset from the beginning
of the section/block/memory-allocation (actual elf sections for *ko's,
and a __start/__stop delineated part of .data for builtins).

N/_index will be used solely to get to __debugs[0] and over to
__debug_sites[N]. It is distinct from a module's numdbgs, which is
used mainly when walking control, and is kept in struct _ddeug_table.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
include/asm-generic/vmlinux.lds.h | 2 ++
include/linux/dynamic_debug.h | 45 +++++++++++++++++++++++++++++++
lib/dynamic_debug.c | 18 ++++++-------
3 files changed, 56 insertions(+), 9 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index f4e861282ed7..11a194fe3cd9 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -340,9 +340,11 @@
#define DYNAMIC_DEBUG_DATA() \
. = ALIGN(8); \
__start___dyndbg_sites = .; \
+ KEEP(*(.gnu.linkonce.dyndbg_site)) \
KEEP(*(__dyndbg_sites)) \
__stop___dyndbg_sites = .; \
__start___dyndbg = .; \
+ KEEP(*(.gnu.linkonce.dyndbg)) \
KEEP(*(__dyndbgs)) \
__stop___dyndbg = .;
#else
diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index c866b4a9760a..a99973221f94 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -113,6 +113,43 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
_DPRINTK_KEY_INIT \
}

+/*
+ * DEFINE_DYNAMIC_DEBUG_TABLE(): This is a special version of
+ * DEFINE_DYNAMIC_DEBUG_METADATA(). It creates a single pair of
+ * header records, which linker scripts place into __dyndbg[0] &
+ * __dyndbg_sites[0].
+ * To allow a robust runtime test for is_dyndbg_header_pair(I,S),
+ * force-initialize S->filename to point back to I, an otherwize
+ * pathological condition.
+ */
+#define DEFINE_DYNAMIC_DEBUG_TABLE() \
+ /* forward decl, allowing loopback pointer */ \
+ __weak struct _ddebug __used __aligned(8) \
+ __section(".gnu.linkonce.dyndbg") \
+ _LINKONCE_dyndbg_header; \
+ __weak struct _ddebug_site __used __aligned(8) \
+ __section(".gnu.linkonce.dyndbg_site") \
+ _LINKONCE_dyndbg_site_header = { \
+ .modname = KBUILD_MODNAME, \
+ .function = KBUILD_MODNAME, \
+ /* forced pointer loopback, for distinction */ \
+ .filename = (void *) &_LINKONCE_dyndbg_header \
+ }; \
+ __weak struct _ddebug __used __aligned(8) \
+ __section(".gnu.linkonce.dyndbg") \
+ _LINKONCE_dyndbg_header = { \
+ .site = &_LINKONCE_dyndbg_site_header, \
+ .format = KBUILD_MODNAME \
+ }
+
+/* The header initializations as a distinguishing predicate */
+#define is_dyndbg_header_pair(iter, sitep) \
+ (sitep == iter->site \
+ && (!iter->lineno) \
+ && (iter->format == sitep->modname) \
+ && (sitep->modname == sitep->function) \
+ && ((void *)sitep->filename == (void *)iter))
+
#ifdef CONFIG_JUMP_LABEL

#ifdef DEBUG
@@ -243,4 +280,12 @@ static inline int dynamic_debug_exec_queries(const char *query, const char *modn

#endif /* !CONFIG_DYNAMIC_DEBUG_CORE */

+#if ((defined(CONFIG_DYNAMIC_DEBUG) || \
+ (defined(CONFIG_DYNAMIC_DEBUG_CORE) && defined(DYNAMIC_DEBUG_MODULE))) \
+ && defined(KBUILD_MODNAME) && !defined(NO_DYNAMIC_DEBUG_TABLE))
+
+/* transparently invoked, except when -DNO_DYNAMIC_DEBUG_TABLE */
+DEFINE_DYNAMIC_DEBUG_TABLE();
+#endif
+
#endif
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index ce134c939f01..e4a22f7b153f 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -1190,8 +1190,7 @@ static int __init dynamic_debug_init(void)
const char *modname = NULL;
char *cmdline;
int ret = 0;
- int site_ct = 0, entries = 0, modct = 0;
- int mod_index = 0;
+ int i, site_ct = 0, modct = 0, mod_index = 0;

if (&__start___dyndbg == &__stop___dyndbg) {
if (IS_ENABLED(CONFIG_DYNAMIC_DEBUG)) {
@@ -1207,10 +1206,9 @@ static int __init dynamic_debug_init(void)
site = site_mod_start = __start___dyndbg_sites;
modname = site->modname;

- for (; iter < __stop___dyndbg; iter++, site++) {
+ for (i = 0; iter < __stop___dyndbg; iter++, site++, i++) {

- BUG_ON(site != iter->site);
- entries++;
+ WARN_ON(site != iter->site);

if (strcmp(modname, site->modname)) {
modct++;
@@ -1219,10 +1217,12 @@ static int __init dynamic_debug_init(void)
site_ct, mod_index, modname);
if (ret)
goto out_err;
- site_ct = 0;
+
modname = site->modname;
iter_mod_start = iter;
site_mod_start = site;
+ mod_index += site_ct;
+ site_ct = 0;
}
site_ct++;
}
@@ -1232,9 +1232,9 @@ static int __init dynamic_debug_init(void)

ddebug_init_success = 1;
vpr_info("%d prdebugs in %d modules, %d KiB in ddebug tables, %d KiB in __dyndbg section, %d KiB in __dyndbg_sites section\n",
- entries, modct, (int)((modct * sizeof(struct ddebug_table)) >> 10),
- (int)((entries * sizeof(struct _ddebug)) >> 10),
- (int)((entries * sizeof(struct _ddebug_site)) >> 10));
+ i, modct, (int)((modct * sizeof(struct ddebug_table)) >> 10),
+ (int)((i * sizeof(struct _ddebug)) >> 10),
+ (int)((i * sizeof(struct _ddebug_site)) >> 10));

/* apply ddebug_query boot param, dont unload tables on err */
if (ddebug_setup_string[0] != '\0') {
--
2.31.1