[PATCH v1] rust: arch/um: Fix loading Rust modules

[FUJITA Tomonori]

Hi, UML maintainers,

On Sun,  4 Oct 2026 21:12:30 +0900
FUJITA Tomonori <tomo at flapping.org> wrote:

> From: FUJITA Tomonori <fujita.tomonori at gmail.com>
> 
> Rust modules fail to load on x86_64 UML:
> 
>   # insmod rnull_mod.ko
>   subarch: rnull_mod: Unknown rela relocation: 9
>   insmod: can't insert 'rnull_mod.ko': invalid module format
> 
> arch/um/Makefile adds -Crelocation-model=pie to KBUILD_RUSTFLAGS. This
> comes after -Crelocation-model=static in the top-level Makefile, so all
> Rust code on UML, both built-in code and modules, is built as PIE. In
> PIE mode, rustc accesses statics in other crates through the GOT and
> emits R_X86_64_GOTPCREL (9) relocations. UML uses the x86 module loader,
> and it does not handle them.
> 
> Remove the flag. Rust code, both built-in code and modules, then uses
> the static relocation model, the same as C code.
> 
> Modules still fail to load with a larger mem=, such as mem=512M,
> because Rust code uses the small code model while C code uses
> -mcmodel=large. This is a separate problem.
> 
> Tested with clang 20 and rustc 1.90: vmlinux builds and boots, and
> rnull_mod.ko loads with mem=32M.
> 
> Reported-by: Zigit Zo <zig at iorw.io>
> Link: https://lore.kernel.org/rust-for-linux/62F5EEA0B866E9E0+20240903130606.292935-1-zig@iorw.io/
> Assisted-by: LLM
> Signed-off-by: FUJITA Tomonori <fujita.tomonori at gmail.com>
> ---
>  arch/um/Makefile | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/um/Makefile b/arch/um/Makefile
> index 937639edc295..c4a99073f014 100644
> --- a/arch/um/Makefile
> +++ b/arch/um/Makefile
> @@ -62,8 +62,6 @@ KBUILD_CFLAGS += $(CFLAGS) $(CFLAGS-y) -D__arch_um__ \
>  	-Din6addr_any=kernel_in6addr_any -Dstrrchr=kernel_strrchr \
>  	-D__close_range=kernel__close_range
>  
> -KBUILD_RUSTFLAGS += -Crelocation-model=pie
> -
>  KBUILD_AFLAGS += $(ARCH_INCLUDE)
>  
>  USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
> 
> base-commit: 2dee3d3adcadcc57d62ba6608cd02f50b0c10264

Gentle ping. The patch still applies cleanly on top of v7.4-rc1. If
it looks fine, could you take it through the UML tree?