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

[Masahiro Yamada]

Hi Josh,


On Wed, Oct 17, 2018 at 1:16 AM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
> 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.


Fair point, but I am confused by the current
STACK_VALIDATION / UNWINDER_ORC logic.

In my understanding, objtool is
an all-in-one object check/manipulation tool.

STACK_VALIDATION and UNWINDER_ORC
is a selection of a sub-command, 'check' or 'orc generate'.

(Correct me if am wrong.)


However, STACK_VALIDATION is still used to
decide whether or not to compile the objtool.


Adding a new symbol OBJTOOL would clarify the logic.



config OBJTOOL
        bool

config STACK_VALIDATION
        bool "Compile-time stack metadata validation"
        depends on HAVE_STACK_VALIDATION
        select OBJTOOL
        ...


config UNWINDER_ORC
        bool "ORC unwinder"
        depends on X86_64
        select OBJTOOL
        ...




-- 
Best Regards
Masahiro Yamada