Re: [PATCH v9 0/4] Generate address range data for built-in modules

[Sami Tolvanen]

Hi Kris,

On Fri, Sep 06, 2024 at 10:45:01AM -0400, Kris Van Hees 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.

v10 looks good to me. I tested x86_64 and arm64 builds with both
GNU and LLVM toolchains, and confirmed that built-in Rust modules
are handled correctly. The code looks reasonable too. For the
series:

Reviewed-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Tested-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>

Sami