vmlinux.lds.h: add HEADERED_SECTION_* macros

These macros elaborate on BOUNDED_SECTION_(PRE|POST)_LABEL macros,
prepending an optional KEEP(.gnu.linkonce##_sec_) reservation, and a
linker-symbol to address it.

This allows a developer to define a header struct (which must fit with
the section's base struct-type), and could contain:

1- fields whose value is common to the entire set of data-records.
   This allows the header & data structs to specialize, complement
   each other, and shrink.

2- an uplink pointer to an organizing struct
   which refs other related/sub data-tables
   header record is addressable via the extern'd header linker-symbol

Once the linker-symbols created by the macro are ref'd extern in code,
that code can compute a record's index (ptr - start) in the "primary"
table, then use it to index into the related/sub tables.  Adding a
primary.map_* field foreach sub-table would then allow deduplication
and remapping of that sub-table.

This is aimed at dyndbg's struct _ddebug __dyndbg[] section, whose 3
columns: function, file, module are 50%, 90%, 100% redundant.  The
module column is fully recoverable after dynamic_debug_init() saves it
to each ddebug_table.module as the builtin __dyndbg[] table is parsed.

Given that those 3 columns use 24/56 of a _ddebug record, a dyndbg=y
kernel with ~5k callsites could reduce kernel memory substantially.
Returning that memory to the kernel buddy-allocator? is then possible.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Link: https://lore.kernel.org/r/20221117171633.923628-3-jim.cromie@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/asm-generic/vmlinux.lds.h

@@ -208,6 +208,21 @@
 
 #define BOUNDED_SECTION(_sec)	 BOUNDED_SECTION_BY(_sec, _sec)
 
+#define HEADERED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
+	_HDR_##_label_	= .;						\
+	KEEP(*(.gnu.linkonce.##_sec_))					\
+	BOUNDED_SECTION_PRE_LABEL(_sec_, _label_, _BEGIN_, _END_)
+
+#define HEADERED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_, _HDR_) \
+	_label_##_HDR_ = .;						\
+	KEEP(*(.gnu.linkonce.##_sec_))					\
+	BOUNDED_SECTION_POST_LABEL(_sec_, _label_, _BEGIN_, _END_)
+
+#define HEADERED_SECTION_BY(_sec_, _label_)				\
+	HEADERED_SECTION_PRE_LABEL(_sec_, _label_, __start, __stop)
+
+#define HEADERED_SECTION(_sec)	 HEADERED_SECTION_BY(_sec, _sec)
+
 #ifdef CONFIG_TRACE_BRANCH_PROFILING
 #define LIKELY_PROFILE()						\
 	BOUNDED_SECTION_BY(_ftrace_annotated_branch, _annotated_branch_profile)