On Fri, Sep 6, 2024 at 11:45 PM Kris Van Hees [off-list ref] wrote:

At build time, create the file modules.builtin.ranges that will hold
address range data of the built-in modules that can be used by tracers.

Especially for tracing applications, it is convenient to be able to
refer to a symbol using a <module name, symbol name> pair and to be able
to translate an address into a <nodule mname, symbol name> pair.  But
that does not work if the module is built into the kernel because the
object files that comprise the built-in module implementation are simply
linked into the kernel image along with all other kernel object files.

This is especially visible when providing tracing scripts for support
purposes, where the developer of the script targets a particular kernel
version, but does not have control over whether the target system has
a particular module as loadable module or built-in module.  When tracing
symbols within a module, referring them by <module name, symbol name>
pairs is both convenient and aids symbol lookup.  But that naming will
not work if the module name information is lost if the module is built
into the kernel on the target system.

Earlier work addressing this loss of information for built-in modules
involved adding module name information to the kallsyms data, but that
required more invasive code in the kernel proper.  This work never did
get merged into the kernel tree.

All that is really needed is knowing whether a given address belongs to
a particular module (or multiple modules if they share an object file).
Or in other words, whether that address falls within an address range
that is associated with one or more modules.

Objects can be identified as belonging to a particular module (or
modules) based on defines that are passed as flags to their respective
compilation commands.  The data found in modules.builtin is used to
determine what modules are built into the kernel proper.  Then,
vmlinux.o.map and vmlinux.map can be parsed in a single pass to generate
a modules.buitin.ranges file with offset range information (relative to
the base address of the associated section) for built-in modules.  This
file gets installed along with the other modules.builtin.* files.

The impact on the kernel build is minimal because everything is done
using a single-pass AWK script.  The generated data size is minimal as
well, (depending on the exact kernel configuration) usually in the range
of 500-700 lines, with a file size of 20-40KB (if all modules are built
in, the file contains about 8000 lines, with a file size of about 285KB).

Changes since v9:
 - Reverted support for optional 4th arg to generator script.
 - Reverted support for optional 6th arg to verifier script.
 - Added modules.builtin.ranges ad vmlinux.o.map to CLEAN_FILES.
 - Fixed support for sparc64.
 - Fixed support for LLVM's lld linker map format.
 - Updated error message when .*.cmd.o cannot be read by verifier script.
 - Added syntax output for verifier script when not enough args are given.
 - Return 1 from verifier if verification fails.

Applied to linux-kbuild.
Thanks!

-- 
Best Regards
Masahiro Yamada