Re: [PATCH] kbuild: lto: fix module versionings mismatch in incremental build

[Kees Cook]

On Tue, Jun 15, 2021 at 02:26:58PM +0800, Lecopzer Chen wrote:
> > On Mon, Jun 14, 2021 at 01:51:09PM +0800, Lecopzer Chen wrote:
> > > When building modules(CONFIG_...=m), I found some of module versions
> > > are incorrect and set to 0.
> > > This can be found in build log for first clean build which shows
> > > 
> > > WARNING: EXPORT symbol "XXXX" [drivers/XXX/XXX.ko] version generation failed, symbol will not be versioned.
> > 
> > I'm doing this, and I don't see the problem:
> > 
> > $ make LLVM=1 LLVM_IAS=1 distclean
> > $ make LLVM=1 LLVM_IAS=1 menuconfig
> > 	*enable LTO*
> > 	*enable a module*
> > $ make LLVM=1 LLVM_IAS=1 -j...
> > 
> > What series of commands (and .config) shows this for you?
> 
> Hi Kees,
> 
> Thanks for you checking.
> 
> After double checking in clean android kernel build, this causes by
> make version.
> (I have build failed in Linux LTO,
> seems it's not well support in contract to android?)
> 
> I knew Google has LTO first in Android and upstream later, and most code
> are same as upstream, so the env here I use Android common kernel for
> easily testing.
> 
> 
> Test env is android common kernel: android12-5.4 [1] with its latest code
> and it builds from build.sh[2]
> 
> $ BUILD_CONFIG=common/build.config.gki.aarch64 build/build.sh
> + make O=.... LLVM=1 LLVM_IAS=1 DEPMOD=depmod -j12 Image modules Image.lz4
> 
> With make set to v3.81, this can be reproduced with CONFIG_TEE=m.
> With version >= 4.2 this is not reproducible.

Ah, very interesting. While there are tests in Makefile for
MAKE_VERSION, if we want to do this, it should likely be extended to
Kconfig, as that's where the initial version tests for things happen. We
could require MAKE_VERSION >= 4.2 for LTO?

-Kees

> 
> 
> Our build env default set make to v3.81, although Android uses hermetic build
> and v4.3 now, but Linux doesn't have such things.
> 
> Maybe we can add build time checking or comment before CFI module versioning
> build rules to avoid anyone struggling with this again:).
> 
> [1] https://android.googlesource.com/kernel/common/+/refs/heads/android12-5.4
> [2] https://android.googlesource.com/kernel/build/+/refs/heads/master
> 
> thanks,
> Lecopzer
> 
> 

-- 
Kees Cook