linux-kernel - Re: [PATCH v2 10/28] kbuild: lto: fix module versioning

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Message-ID: <20200908182308.GA1227805@google.com>
Date:   Tue, 8 Sep 2020 11:23:08 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        x86@...nel.org
Subject: Re: [PATCH v2 10/28] kbuild: lto: fix module versioning

On Thu, Sep 03, 2020 at 03:11:54PM -0700, Kees Cook wrote:
> On Thu, Sep 03, 2020 at 01:30:35PM -0700, Sami Tolvanen wrote:
> > With CONFIG_MODVERSIONS, version information is linked into each
> > compilation unit that exports symbols. With LTO, we cannot use this
> > method as all C code is compiled into LLVM bitcode instead. This
> > change collects symbol versions into .symversions files and merges
> > them in link-vmlinux.sh where they are all linked into vmlinux.o at
> > the same time.
> > 
> > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> 
> The only thought I have here is I wonder if this change could be made
> universally instead of gating on LTO? (i.e. is it noticeably slower to
> do it this way under non-LTO?)

I don't think it's noticeably slower, but keeping the version information
in object files when possible is cleaner, in my opinion.

Sami