Re: [PATCH RFC] kbuild: Prevent compiler mismatch with external modules

[Josh Poimboeuf]

On Thu, Jan 28, 2021 at 12:24:45PM -0800, Linus Torvalds wrote:
> On Thu, Jan 28, 2021 at 12:08 PM Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> >
> > Add a check for compiler mismatch, but only check the major version.
> 
> I think this is wrong for multiple reasons.
> 
> The most fundamental reason is that it's pointless and doesn't
> actually do what you claim it does.
> 
> Just doing a "make oldconfig" will reset the CONFIG_xyz_VERSION to
> whatever is installed, and now your check doesn't actually do
> anything, since you're not actually checking what the kernel was
> compiled with!

Huh?  Why would you do a "make oldconfig" on a distro-released kernel
before building an OOT module?

Usually you build an OOT module against /lib/modules/$(uname -r)/build,
and the .config isn't even writable.

> So I think that check is pointless and entirely misleading. It doesn't
> do what you want it to do, and what you claim it does.

It's not a magic bullet, but doesn't it catch the vast majority of use
cases?  Which makes it a lot better than what we have now (nothing).

> I'm not convinced about the whole magic vs minor argument either. The
> whole "new compiler features" thing is a red herring - even if you do
> have new compiler features, that in itself has very little to do with
> whether the resulting object files are compatible or not.

Hm?  Are you saying the check is too strict, since GCC9 binaries _might_
be compatible with GCC10?

-- 
Josh