linux-kernel - Re: [PATCH 2/3] objtool: move libelf check out of top Makefile

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Date:   Tue, 16 Oct 2018 11:15:36 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        Borislav Petkov <bp@...e.de>, Sam Ravnborg <sam@...nborg.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] objtool: move libelf check out of top Makefile

On Wed, Oct 17, 2018 at 12:51:40AM +0900, Masahiro Yamada wrote:
> > ifdef CONFIG_UNWINDER_ORC
> >
> > chk_unwinder_orc = echo "int main() {}" | $(HOSTCC) -xc -o /dev/null -lelf -
> > msg_unwinder_orc = "Cannot build objtool to generate ORC metadata for CONFIG_UNWINDER_ORC=y.  " \
> >                        "Please install libelf-dev, libelf-devel or elfutils-libelf-devel."
> > toolcheck-$(CONFIG_UNWINDER_ORC) += unwinder_orc
> >
> > else
> >
> > chk_stack_validation = echo "int main() {}" | $(HOSTCC) -xc -o /dev/null -lelf -
> > msg_stack_validation = "Cannot build objtool for CONFIG_STACK_VALIDATION=y.  " \
> >                        "Please install libelf-dev, libelf-devel or elfutils-libelf-devel."
> > toolcheck-$(CONFIG_STACK_VALIDATION) += stack_validation
> >
> > endif
> >
> >
> > What do you think?
> 
> 
> It is ugly.
> 
> Do you need such detailed information like ORC metadata stuff here?
> 
> This Makefile aims to error out, showing why the build failed.
> That's it.

Yeah, it is kind of ugly.  But the "showing why the build failed" part
is important.  I was trying to give the user a clear error message,
similar to what we have today.

Without context, the user won't know what objtool is, or why it needs to
be built.

If we have just a single error message for all cases, it should at least
mention the config option.  Like

   "Cannot build objtool for CONFIG_STACK_VALIDATION."

But then, most users will only have that enabled because of ORC.  So an
ORC-specific message would be more appropriate in most cases.

So maybe it can just be something more vague:

msg_stack_validation = "Cannot build objtool for CONFIG_UNWINDER_ORC and/or CONFIG_STACK_VALIDATION.  " \
		       "Please install libelf-dev, libelf-devel or elfutils-libelf-devel."

That would probably be good enough.  Then we could drop the ugly ifdef.

-- 
Josh