MO flag not working properly for building out-of-tree modules, tries to replace the Makefile in the kernel's source.

MO flag not working properly for building out-of-tree modules, tries to replace the Makefile in the kernel's source.

[Anish Rashinkar]

I have a few projects in which I am writing kernel modules. I use the
MO flag to put the build artifacts in a separate directory. A simple
make command with MO would look like -
make -C /lib/modules/$(uname -r)/build M=$PWD MO=$PWD/build/ modules

This used to work for me, and still works for me if I boot into the
6.19.10-300 kernel that Fedora preserved. I was on 7.0.X for a while
on Arch Linux and it worked there too. I am not sure but I might have
tried compiling my projects (if so, compiled successfully) on Fedora
Atomic that I was trying out, which should have been on 7.0 or 7.1. As
of the latest kernels on both Arch Linux and Fedora (7.1.3-201 on
Fedora), I encounter an error when using the MO flag.

make -C /lib/modules/7.1.3-201.fc44.x86_64/build M=/home/anish/temp
MO=/home/anish/temp/build modules
make[1]: Entering directory '/usr/src/kernels/7.1.3-201.fc44.x86_64'
make[2]: Entering directory '/home/anish/temp/build'
/bin/sh: line 1: /usr/src/kernels/7.1.3-201.fc44.x86_64/.tmp_Makefile:
Permission denied
make[3]: *** [/usr/src/kernels/7.1.3-201.fc44.x86_64/Makefile:705:
/usr/src/kernels/7.1.3-201.fc44.x86_64/Makefile] Error 1
make[2]: *** [/usr/src/kernels/7.1.3-201.fc44.x86_64/Makefile:248:
__sub-make] Error 2
make[2]: Leaving directory '/home/anish/temp/build'
make[1]: *** [Makefile:248: __sub-make] Error 2
make[1]: Leaving directory '/usr/src/kernels/7.1.3-201.fc44.x86_64'
make: *** [Makefile:6: all] Error 2

If I grant root privileges to make, it will replace the Makefile at
/usr/src/kernels/7.1.3-201.fc44.x86_64/Makefile with its own Makefile
wrapper.

If I don't use the MO flag, then the build succeeds and all the build
artifacts are stored in my source directory.

I am new to Linux programming and especially bug reporting like this.
I had raised this issue on Fedora Discussions with no one to properly
respond that helps the problem. This issue should have appeared
relatively recently. Please let me know if this is a bug or something
changed with how it should be used, because the official guides state
the same way I mentioned in writing a Makefile and using the MO flag.
And with what I can comprehend and with what all LLMs are saying, this
is a bug.

Re: MO flag not working properly for building out-of-tree modules, tries to replace the Makefile in the kernel's source.

[Nicolas Schier]

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[Cc: nathan]

On Thu, Jul 16, 2026 at 11:04:23PM +0530, Anish Rashinkar wrote:
> I have a few projects in which I am writing kernel modules. I use the
> MO flag to put the build artifacts in a separate directory. A simple
> make command with MO would look like -
> make -C /lib/modules/$(uname -r)/build M=$PWD MO=$PWD/build/ modules
> 
> This used to work for me, and still works for me if I boot into the
> 6.19.10-300 kernel that Fedora preserved. I was on 7.0.X for a while
> on Arch Linux and it worked there too. I am not sure but I might have
> tried compiling my projects (if so, compiled successfully) on Fedora
> Atomic that I was trying out, which should have been on 7.0 or 7.1. As
> of the latest kernels on both Arch Linux and Fedora (7.1.3-201 on
> Fedora), I encounter an error when using the MO flag.
> 
> make -C /lib/modules/7.1.3-201.fc44.x86_64/build M=/home/anish/temp
> MO=/home/anish/temp/build modules
> make[1]: Entering directory '/usr/src/kernels/7.1.3-201.fc44.x86_64'
> make[2]: Entering directory '/home/anish/temp/build'
> /bin/sh: line 1: /usr/src/kernels/7.1.3-201.fc44.x86_64/.tmp_Makefile:
> Permission denied
> make[3]: *** [/usr/src/kernels/7.1.3-201.fc44.x86_64/Makefile:705:
> /usr/src/kernels/7.1.3-201.fc44.x86_64/Makefile] Error 1
> make[2]: *** [/usr/src/kernels/7.1.3-201.fc44.x86_64/Makefile:248:
> __sub-make] Error 2
> make[2]: Leaving directory '/home/anish/temp/build'
> make[1]: *** [Makefile:248: __sub-make] Error 2
> make[1]: Leaving directory '/usr/src/kernels/7.1.3-201.fc44.x86_64'
> make: *** [Makefile:6: all] Error 2
> 
> If I grant root privileges to make, it will replace the Makefile at
> /usr/src/kernels/7.1.3-201.fc44.x86_64/Makefile with its own Makefile
> wrapper.
> 
> If I don't use the MO flag, then the build succeeds and all the build
> artifacts are stored in my source directory.
> 
> I am new to Linux programming and especially bug reporting like this.
> I had raised this issue on Fedora Discussions with no one to properly
> respond that helps the problem. This issue should have appeared
> relatively recently. Please let me know if this is a bug or something
> changed with how it should be used, because the official guides state
> the same way I mentioned in writing a Makefile and using the MO flag.
> And with what I can comprehend and with what all LLMs are saying, this
> is a bug.

Thanks for the report!  This is definitely a bug and I can reproduce 
this.  I'll have a look at it tomorrow.

Kind regards,
Nicolas

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Re: Re: MO flag not working properly for building out-of-tree modules, tries to replace the Makefile in the kernel's source.

[logafinl]

Hello,

I was running into this issue myself and used bisect to pinpoint
c9bb03ac2c66bc5aa81b51ea0792477524c2763a as the commit that introduced the bug.
I don't have enough experience in Kbuild to understand what in that commit
causing the problem, unfortunately.

Thanks,
Logan Finley

Re: Re: MO flag not working properly for building out-of-tree modules, tries to replace the Makefile in the kernel's source.

[Nathan Chancellor]

On Wed, Jul 29, 2026 at 03:32:22PM -0700, logafinl wrote:
> Hello,
> 
> I was running into this issue myself and used bisect to pinpoint
> c9bb03ac2c66bc5aa81b51ea0792477524c2763a as the commit that introduced the bug.
> I don't have enough experience in Kbuild to understand what in that commit
> causing the problem, unfortunately.

Does

  https://git.kernel.org/kbuild/c/39d6e68f50f4a444a6ba23b9ea2eaee806601c6d

resolve this issue for you?

-- 
Cheers,
Nathan

Re: Re: MO flag not working properly for building out-of-tree modules, tries to replace the Makefile in the kernel's source.

[Logan Finley]

On Wed, Jul 29, 2026 at 04:12:42PM -0700, Nathan Chancellor wrote:
> Does
> 
>   https://git.kernel.org/kbuild/c/39d6e68f50f4a444a6ba23b9ea2eaee806601c6d
> 
> resolve this issue for you?

Yes, it does. I apologize; I didn't see that there was already a fix.

Thanks,
Logan Finley

Re: Re: MO flag not working properly for building out-of-tree modules, tries to replace the Makefile in the kernel's source.

[Nathan Chancellor]

On Wed, Jul 29, 2026 at 04:36:41PM -0700, Logan Finley wrote:
> On Wed, Jul 29, 2026 at 04:12:42PM -0700, Nathan Chancellor wrote:
> > Does
> > 
> >   https://git.kernel.org/kbuild/c/39d6e68f50f4a444a6ba23b9ea2eaee806601c6d
> > 
> > resolve this issue for you?
> 
> Yes, it does. I apologize; I didn't see that there was already a fix.

No worries, it is a little hidden away. Thanks a lot for confirming that
it works.

-- 
Cheers,
Nathan