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From:   Kees Cook <keescook@chromium.org>
Date:   Tue, 27 Mar 2018 20:26:41 -0700
Message-ID: <CAGXu5jJBePX5b6zbSa5rM=ouFgSeLvqxEg3kxTViKdnOukmnGA@mail.gmail.com>
Subject: Re: [PATCH v2 01/21] kbuild: remove kbuild cache
To:     Masahiro Yamada <yamada.masahiro@socionext.com>
Cc:     linux-kbuild <linux-kbuild@vger.kernel.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Ulf Magnusson <ulfalizer@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Luis R . Rodriguez" <mcgrof@kernel.org>,
        Nicolas Pitre <nico@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Mon, Mar 26, 2018 at 10:29 PM, Masahiro Yamada
<yamada.masahiro@socionext.com> wrote:
> The kbuild cache was introduced to remember the result of shell
> commands, some of which are expensive to compute, such as
> $(call cc-option,...).
>
> However, this turned out not so clever as I had first expected.
> Actually, it is problematic.  For example, "$(CC) -print-file-name"
> is cached.  If the compiler is updated, the stale search path causes
> build error, which is difficult to figure out.  Another problem
> scenario is cache files could be touched while install targets are
> running under the root permission.  We can patch them if desired,
> but the build infrastructure is getting uglier and uglier.
>
> Now, we are going to move compiler flag tests to the configuration
> phase.  If this is completed, the result of compiler tests will be
> naturally cached in the .config file.  We will not have performance
> issues of incremental building since this testing only happens at
> Kconfig time.
>
> To start this work with a cleaner code base, remove the kbuild
> cache first.
>
> Revert the following commits:
> Commit 9a234a2e3843 ("kbuild: create directory for make cache only when necessary")
> Commit e17c400ae194 ("kbuild: shrink .cache.mk when it exceeds 1000 lines")
> Commit 4e56207130ed ("kbuild: Cache a few more calls to the compiler")
> Commit 3298b690b21c ("kbuild: Add a cache for generated variables")
>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

-- 
Kees Cook
Pixel Security