linux-kernel - Re: upgrade the orphan section warning to a hard link error

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Message-ID: <202210171230.CC40461C@keescook>
Date:   Mon, 17 Oct 2022 12:32:39 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Nathan Chancellor <nathan@...nel.org>
Cc:     "Li, Xin3" <xin3.li@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "H.Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, llvm@...ts.linux.dev,
        linux-kbuild@...r.kernel.org
Subject: Re: upgrade the orphan section warning to a hard link error

On Mon, Oct 17, 2022 at 11:26:47AM -0700, Nathan Chancellor wrote:
> It might be interesting to turn orphan sections into an error if
> CONFIG_WERROR is set. Perhaps something like the following (FYI, not
> even compile tested)?
> 
> diff --git a/Makefile b/Makefile
> index 0837445110fc..485f47fc2c07 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1119,7 +1119,7 @@ endif
>  # We never want expected sections to be placed heuristically by the
>  # linker. All sections should be explicitly named in the linker script.
>  ifdef CONFIG_LD_ORPHAN_WARN
> -LDFLAGS_vmlinux += --orphan-handling=warn
> +LDFLAGS_vmlinux += --orphan-handling=$(if $(CONFIG_WERROR),error,warn)
>  endif

Yes, this is much preferred.

> Outright turning the warning into an error with no escape hatch might be
> too aggressive, as we have had these warnings triggered by new compiler
> generated sections, such as in commit 848378812e40 ("vmlinux.lds.h:
> Handle clang's module.{c,d}tor sections"). Unconditionally breaking the
> build in these situations is unfortunate but the warnings do need to be
> dealt with so I think having it error by default with the ability to
> opt-out is probably worth doing. I do not have a strong opinion though.

Correct; the mandate from Linus (disregarding his addition of
CONFIG_WERROR for all*config builds), is that we should avoid breaking
builds. It wrecks bisection, it causes problems across compiler versions,
etc.

So, yes, only on CONFIG_WERROR=y.

-- 
Kees Cook