[PATCH v2 5/5] dyndbg.lds.S: fix lost dyndbg sections in modules

[Jim Cromie]

With CONFIG_DRM_USE_DYNAMIC_DEBUG=y, several build configs had
problems with __dyndbg* sections getting lost in drm drivers.  Fix
this by following the model demonstrated in codetag.lds.h.

Introduce include/asm-generic/dyndbg.lds.h, to bundle dynamic-debug's
multiple sections together, into 2 macros:

vmlinux.lds.h DATA_DATA: move the 2 BOUNDED_SECTION_BY(__dyndbg*)
calls into dyndbg.lds.h DYNDBG_SECTIONS(). vmlinux.lds.h now includes
the new file and calls the new macro.

MOD_DYNDBG_SECTIONS declares the 2 BOUNDED_SECTION_BY calls, but wraps
them with output section syntax to keep them as known and separate ELF
sections in the module.ko.  The KEEP fixes the lost section.

dyndbg.lds.h includes (reuses) bounded-section.lds.h

scripts/module.lds.S: now calls MOD_DYNDBG_SECTIONS right before the
CODETAG macro (consistent with their placements in vmlinux.lds.h), and
also includes dyndbg.lds.h

This isolates vmlinux.lds.h from further __dyndbg section additions.

CC: Arnd Bergmann <arnd@xxxxxxxx>
Signed-off-by: Jim Cromie <jim.cromie@xxxxxxxxx>
---

v2 - Address linker script review feedback for relocatable modules.

v1 of MOD_DYNDBG_SECTIONS() used the BOUNDED_SECTION_BY() macro, which
proved problematic for kernel modules for two reasons:

1. Unwanted Empty Sections:
   BOUNDED_SECTION_BY() automatically generates `__start` and `__stop`
   symbols. When applied to `MOD_DYNDBG_SECTIONS()`, the linker assumes
   the sections are populated due to the symbol definitions, forcing an
   empty `__dyndbg` and `__dyndbg_classes` output section in every
   compiled module, even those without dynamic debug configuration.
   Since the module loader uses `section_objs()` to locate data via
   ELF headers instead of relying on `__start`/`__stop` symbols, these
   assignments are completely unnecessary.

2. Non-zero Output Addresses:
   During relocatable linking (e.g., `ld.bfd -r`), omitting an explicit
   base address causes the section to inherit the current location
   counter. This results in non-zero sh_addr values in `.ko` files,
   which is confusing, degrades compressibility, and can cause issues
   with external tools parsing the ELF.

Fix both issues by dropping `BOUNDED_SECTION_BY()` in favor of a simple
`KEEP(*(...))` constraint and explicitly defining the sections with a `0`
base address: `__dyndbg 0 : ALIGN(8) { ... }`.
---
 MAINTAINERS                       |  1 +
 include/asm-generic/dyndbg.lds.h  | 19 +++++++++++++++++++
 include/asm-generic/vmlinux.lds.h |  6 ++----
 scripts/module.lds.S              |  2 ++
 4 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/MAINTAINERS b/MAINTAINERS
index 882214b0e7db..54bb257b60ee 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9070,6 +9070,7 @@ DYNAMIC DEBUG
 M:	Jason Baron <jbaron@xxxxxxxxxx>
 M:	Jim Cromie <jim.cromie@xxxxxxxxx>
 S:	Maintained
+F:	include/asm-generic/dyndbg.lds.h
 F:	include/linux/dynamic_debug.h
 F:	lib/dynamic_debug.c
 F:	lib/test_dynamic_debug.c
diff --git a/include/asm-generic/dyndbg.lds.h b/include/asm-generic/dyndbg.lds.h
new file mode 100644
index 000000000000..844f1cb82020
--- /dev/null
+++ b/include/asm-generic/dyndbg.lds.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __ASM_GENERIC_DYNDBG_LDS_H
+#define __ASM_GENERIC_DYNDBG_LDS_H
+
+#include <asm-generic/bounded_sections.lds.h>
+#define DYNDBG_SECTIONS()					\
+	. = ALIGN(8);						\
+	BOUNDED_SECTION_BY(__dyndbg, ___dyndbg)			\
+	BOUNDED_SECTION_BY(__dyndbg_classes, ___dyndbg_classes)
+
+#define MOD_DYNDBG_SECTIONS()						\
+	__dyndbg 0 : ALIGN(8) {						\
+		KEEP(*(__dyndbg))					\
+	}								\
+	__dyndbg_classes 0 : ALIGN(8) {					\
+		KEEP(*(__dyndbg_classes))				\
+	}
+
+#endif /* __ASM_GENERIC_DYNDBG_LDS_H */
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index d9d2eb708355..54897d742c6c 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -332,6 +332,7 @@
 /*
  * .data section
  */
+#include <asm-generic/dyndbg.lds.h>
 #define DATA_DATA							\
 	*(.xiptext)							\
 	*(DATA_MAIN)							\
@@ -345,10 +346,7 @@
 	*(.data..do_once)						\
 	STRUCT_ALIGN();							\
 	*(__tracepoints)						\
-	/* implement dynamic printk debug */				\
-	. = ALIGN(8);							\
-	BOUNDED_SECTION_BY(__dyndbg_classes, ___dyndbg_classes)		\
-	BOUNDED_SECTION_BY(__dyndbg, ___dyndbg)				\
+	DYNDBG_SECTIONS()						\
 	CODETAG_SECTIONS()						\
 	LIKELY_PROFILE()		       				\
 	BRANCH_PROFILE()						\
diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index b62683061d79..2e62dc5bd5d4 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -10,6 +10,7 @@
 #endif
 
 #include <asm-generic/codetag.lds.h>
+#include <asm-generic/dyndbg.lds.h>
 
 SECTIONS {
 	/DISCARD/ : {
@@ -61,6 +62,7 @@ SECTIONS {
 		*(.rodata..L*)
 	}
 
+	MOD_DYNDBG_SECTIONS()
 	MOD_SEPARATE_CODETAG_SECTIONS()
 }
 

-- 
2.54.0