ARMv7 Linux + Rust doesn't boot when compiling with only LLVM=1

[Rudraksha Gupta]

On 12/16/25 06:41, Christian Schrefl wrote:
> On 12/14/25 12:54 AM, Rudraksha Gupta wrote:
>> Hello all,
>>
>>
>> I have the following problem: https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/issues/2635
>>
>>
>> In short, what is happening is the following:
>>
>>
>> - The kernel boots and outputs via UART when I build the kernel with the following:
>>
>> make LLVM=1 ARCH="$arm" CC="${CC:-gcc}"
>>
>>
>> - The kernel doesn't boot and there is no output via UART when I build the kernel with the following:
>>
>> make LLVM=1 ARCH="$arm"
>>
>>
>> The only difference being: CC="${CC:-gcc}". Is this expected? I think this was present in the Linux kernel ever since Rust was enabled for ARMv7, and I never encountered it because postmarketOS was originally building the first way.
>
> I've managed to the get the build setup for qemu-armv7. For some reason
> I could not get past the initrd even on kernels that are supposed to work,
> but I think that is unrelated (and not a kernel issue).

Yep, I just got qemu-arm working to drop into a debug shell for now. I 
have to look into why other things aren't behaving nicely (but that's a 
problem for later me :P). For now, it seems to demonstrate the problem 
nicely:

https://gitlab.postmarketos.org/postmarketOS/pmbootstrap/-/issues/2635#note_521740


> On the linux-next kernel I didn't get any output on the console from qemu so I
> think I've reproduced the issue. Changing CONFIG_RUST=n did not change the behavior.
>
> So I this is almost certainly a LLVM/clang issue and not a Rust issue. I'll try to
> do a bit more digging, but I'm not sure how much I'll get done.

Did a little more testing in addition to the testing in the gitlab issue 
mentioned above:

- Removed Rust configs from linux-next/pmos.config -> didn't boot on 
qemu-arm and my phone

- Then I removed Rust dependencies from linux-next/APKBUILD -> didn't 
boot on qemu-arm and my phone

- used linux-stable instead of linux-next -> booted on qemu-arm to a 
debug shell

linux-stable is built via gcc:
https://gitlab.alpinelinux.org/alpine/aports/-/blob/master/community/linux-stable/APKBUILD#L179

linux-next is built via clang:
https://gitlab.postmarketos.org/postmarketOS/pmaports/-/blob/master/device/testing/linux-next/APKBUILD#L68



>
> Adding the LLVM and ARM lists to this conversation.
>
> Cheers,
> Christian