linux-kernel - Re: [PATCH v4 05/14] modpost: extract symbol versions from *.cmd files

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Message-ID: <CABCJKucG4dOLm9pPWrACc+YReBXG4GNM9V0djsy0YTp8pRGmaw@mail.gmail.com>
Date:   Mon, 9 May 2022 14:52:21 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nicolas Schier <nicolas@...sle.eu>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-modules@...r.kernel.org, linux-s390@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v4 05/14] modpost: extract symbol versions from *.cmd files

On Sun, May 8, 2022 at 12:09 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
>
> Currently, CONFIG_MODVERSIONS needs extra link to embed the symbol
> versions into ELF objects. Then, modpost extracts the version CRCs
> from them.
>
> The following figures show how it currently works, and how I am trying
> to change it.
>
> Current implementation
> ======================
>                                                            |----------|
>                  embed CRC      -------------------------->| final    |
>        $(CC)       $(LD)       /  |---------|              | link for |
>        -----> *.o -------> *.o -->| modpost |              | vmlinux  |
>       /              /            |         |-- *.mod.c -->| or       |
>      / genksyms     /             |---------|              | module   |
>   *.c ------> *.symversions                                |----------|
>
> Genksyms outputs the calculated CRCs in the form of linker script
> (*.symversions), which is used by $(LD) to update the object.
>
> If CONFIG_LTO_CLANG=y, the build process is much more complex. Embedding
> the CRCs is postponed until the LLVM bitcode is converted into ELF,
> creating another intermediate *.prelink.o.
>
> However, this complexity is unneeded. There is no reason why we must
> embed version CRCs in objects so early.
>
> There is final link stage for vmlinux (scripts/link-vmlinux.sh) and
> modules (scripts/Makefile.modfinal). We can link CRCs at the very last
> moment.
>
> New implementation
> ==================
>                                                            |----------|
>                    --------------------------------------->| final    |
>        $(CC)      /    |---------|                         | link for |
>        -----> *.o ---->|         |                         | vmlinux  |
>       /                | modpost |--- .vmlinux.export.c -->| or       |
>      / genksyms        |         |--- *.mod.c ------------>| module   |
>   *.c ------> *.cmd -->|---------|                         |----------|
>
> Pass the symbol versions to modpost as separate text data, which are
> available in *.cmd files.
>
> This commit changes modpost to extract CRCs from *.cmd files instead of
> from ELF objects.
>
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> Reviewed-by: Nicolas Schier <nicolas@...sle.eu>
> Tested-by: Nathan Chancellor <nathan@...nel.org>

This looks good to me, it's great to get rid of the .symversions files!

Reviewed-by: Sami Tolvanen <samitolvanen@...gle.com>

Sami