linux-kernel - Re: [PATCH v2] objtool: move libelf detection to Kconfig from Makefile

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Message-ID: <CAK7LNASM+81JkGu5-z8GRZ9+sRyd55gO0G0M5c54zRhE5JhE3g@mail.gmail.com>
Date:   Tue, 10 Jul 2018 12:47:49 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>
Subject: Re: [PATCH v2] objtool: move libelf detection to Kconfig from Makefile

Hi.


2018-07-10 11:29 GMT+09:00 Josh Poimboeuf <jpoimboe@...hat.com>:
> On Tue, Jul 10, 2018 at 10:35:16AM +0900, Masahiro Yamada wrote:
>> Currently, users are allowed to enable STACK_VALIDATION regardless
>> of the compiler capability.  The top-level Makefile warns or breaks
>> the build if it turns out that the host compiler cannot link libelf.
>>
>> Move the libelf test to Kconfig so that users can enable the feature
>> only when the host compiler can build the objtool.  The ugly check
>> in the Makefile will go away.
>>
>> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
>> Acked-by: Josh Poimboeuf <jpoimboe@...hat.com>
>
> Actually, looking at this again, I want to rescind my ack.
>
> This patches changes the behavior in a subtle (but important) way.
>
> Before, if I did "make defconfig", it would *always* choose the ORC
> unwinder.  Then, if I didn't have libelf-devel, the build would fail and
> it would tell me what I need to install.
>
> But now with this patch, if I do "make defconfig", the generated config
> actually changes based on what I happen to have installed on my build
> system.  If I don't have libelf-devel, then it silently chooses the
> non-default unwinder (frame pointer).
>
> This is a subtle difference, but IMO it's dangerous, because it will
> silently enable the frame pointer unwinder for the majority of new
> systems, even though it's not the default.
>
> I strongly prefer the original behavior, because we really want to
> encourage people to use the ORC unwinder, even if that means they have
> to install a package to build it.
>
> Also -- in general -- I suspect that silently changing the defaults like
> this will create a lot of bad surprises.  The output of "make defconfig"
> should be predictable and not dependent on what RPMs I happen to have
> installed.



Actually, we had similar discussion for stack protector.


First, Kees liked to let the build fail
instead of disabling the stack protector silently:

https://lkml.org/lkml/2018/2/9/697



Linus said:
But yes, I also reacted to your earlier " It can't silently rewrite it
to _REGULAR because the compiler support for _STRONG regressed."
Because it damn well can. If the compiler doesn't support
-fstack-protector-strong, we can just fall back on -fstack-protector.
Silently. No extra crazy complex logic for that either.

(https://lkml.org/lkml/2018/2/10/281)



I hope this is the same pattern.



-- 
Best Regards
Masahiro Yamada