Re: [PATCH v9 2/4] kbuild: generate offset range data for builtin modules

[Kris Van Hees]

On Mon, Sep 02, 2024 at 04:26:48PM +0900, Masahiro Yamada wrote:
> On Sat, Aug 24, 2024 at 12:38???PM Kris Van Hees <kris.van.hees@xxxxxxxxxx> wrote:
> >
> > Create file module.builtin.ranges that can be used to find where
> > built-in modules are located by their addresses. This will be useful for
> > tracing tools to find what functions are for various built-in modules.
> >
> > The offset range data for builtin modules is generated using:
> >  - modules.builtin: associates object files with module names
> >  - vmlinux.map: provides load order of sections and offset of first member
> >     per section
> >  - vmlinux.o.map: provides offset of object file content per section
> >  - .*.cmd: build cmd file with KBUILD_MODFILE
> >
> > The generated data will look like:
> >
> > .text 00000000-00000000 = _text
> > .text 0000baf0-0000cb10 amd_uncore
> > .text 0009bd10-0009c8e0 iosf_mbi
> > ...
> > .text 00b9f080-00ba011a intel_skl_int3472_discrete
> > .text 00ba0120-00ba03c0 intel_skl_int3472_discrete intel_skl_int3472_tps68470
> > .text 00ba03c0-00ba08d6 intel_skl_int3472_tps68470
> > ...
> > .data 00000000-00000000 = _sdata
> > .data 0000f020-0000f680 amd_uncore
> >
> > For each ELF section, it lists the offset of the first symbol.  This can
> > be used to determine the base address of the section at runtime.
> >
> > Next, it lists (in strict ascending order) offset ranges in that section
> > that cover the symbols of one or more builtin modules.  Multiple ranges
> > can apply to a single module, and ranges can be shared between modules.
> >
> > The CONFIG_BUILTIN_MODULE_RANGES option controls whether offset range data
> > is generated for kernel modules that are built into the kernel image.
> >
> > How it works:
> >
> >  1. The modules.builtin file is parsed to obtain a list of built-in
> >     module names and their associated object names (the .ko file that
> >     the module would be in if it were a loadable module, hereafter
> >     referred to as <kmodfile>).  This object name can be used to
> >     identify objects in the kernel compile because any C or assembler
> >     code that ends up into a built-in module will have the option
> >     -DKBUILD_MODFILE=<kmodfile> present in its build command, and those
> >     can be found in the .<obj>.cmd file in the kernel build tree.
> >
> >     If an object is part of multiple modules, they will all be listed
> >     in the KBUILD_MODFILE option argument.
> >
> >     This allows us to conclusively determine whether an object in the
> >     kernel build belong to any modules, and which.
> >
> >  2. The vmlinux.map is parsed next to determine the base address of each
> >     top level section so that all addresses into the section can be
> >     turned into offsets.  This makes it possible to handle sections
> >     getting loaded at different addresses at system boot.
> >
> >     We also determine an 'anchor' symbol at the beginning of each
> >     section to make it possible to calculate the true base address of
> >     a section at runtime (i.e. symbol address - symbol offset).
> >
> >     We collect start addresses of sections that are included in the top
> >     level section.  This is used when vmlinux is linked using vmlinux.o,
> >     because in that case, we need to look at the vmlinux.o linker map to
> >     know what object a symbol is found in.
> >
> >     And finally, we process each symbol that is listed in vmlinux.map
> >     (or vmlinux.o.map) based on the following structure:
> >
> >     vmlinux linked from vmlinux.a:
> >
> >       vmlinux.map:
> >         <top level section>
> >           <included section>  -- might be same as top level section)
> >             <object>          -- built-in association known
> >               <symbol>        -- belongs to module(s) object belongs to
> >               ...
> >
> >     vmlinux linked from vmlinux.o:
> >
> >       vmlinux.map:
> >         <top level section>
> >           <included section>  -- might be same as top level section)
> >             vmlinux.o         -- need to use vmlinux.o.map
> >               <symbol>        -- ignored
> >               ...
> >
> >       vmlinux.o.map:
> >         <section>
> >             <object>          -- built-in association known
> >               <symbol>        -- belongs to module(s) object belongs to
> >               ...
> >
> >  3. As sections, objects, and symbols are processed, offset ranges are
> >     constructed in a straight-forward way:
> >
> >       - If the symbol belongs to one or more built-in modules:
> >           - If we were working on the same module(s), extend the range
> >             to include this object
> >           - If we were working on another module(s), close that range,
> >             and start the new one
> >       - If the symbol does not belong to any built-in modules:
> >           - If we were working on a module(s) range, close that range
> >
> > Signed-off-by: Kris Van Hees <kris.van.hees@xxxxxxxxxx>
> > Reviewed-by: Nick Alcock <nick.alcock@xxxxxxxxxx>
> > Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> > Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> > ---
> >     Changes since v8:
> >      - Added support for built-in Rust modules.
> >      - Added optional 4th argument to specify kernel build directory.
> 
> 
> When is the 4th parameter useful?
> 
> The map file knows all object paths.
> 
> The object file and its .*.cmd file exist in the same directory.
> 
> I did not find a use-case where the kdir prefix plays a role.
> 
> Did I miss something?

I added it as an optional parameter in case there is a reason to run the
script manually and the kernel was built using O=<objdir>.

	Kris