[PATCH 1/2] rust: arch/um: use 'static' relocation model for uml modules

[PATCH 1/2] rust: arch/um: use 'static' relocation model for uml modules

[Zigit Zo]

In the x86_64 UML, kernel modules compiled with Rust will currently
generate some R_X86_64_GOTPCREL relocations, which will then be rejected
by the kernel.

And because of the kernel modules of UML are only got handled by the UML
itself, relocation model 'static' can work as expected other than 'pie'.
---
 arch/um/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index 00b63bac5eff..b04b1d4d6dfe 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -63,7 +63,7 @@ KBUILD_CFLAGS += $(CFLAGS) $(CFLAGS-y) -D__arch_um__ \
 	-Din6addr_loopback=kernel_in6addr_loopback \
 	-Din6addr_any=kernel_in6addr_any -Dstrrchr=kernel_strrchr
 
-KBUILD_RUSTFLAGS += -Crelocation-model=pie
+KBUILD_RUSTFLAGS_KERNEL += -Crelocation-model=pie
 
 KBUILD_AFLAGS += $(ARCH_INCLUDE)
 
-- 
2.46.0

[PATCH 2/2] rust: arch/um: use 'large' code model for uml

[Zigit Zo]

The Rust modules compiled against UML should be 'large', without this
the compiler will generate some R_X86_64_32S relocations, but UML is
running at userspace with signed-address larger than zero, therefore
makes '__write_relocate_add' treating the relocation as overflowed.
---
 arch/um/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index b04b1d4d6dfe..c2a9f2102ba7 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -32,6 +32,7 @@ endif
 
 ifdef CONFIG_64BIT
 	KBUILD_CFLAGS += -mcmodel=large
+	KBUILD_RUSTFLAGS += -Ccode-model=large
 endif
 
 HOST_DIR := arch/$(HEADER_ARCH)
-- 
2.46.0

Re: [PATCH 1/2] rust: arch/um: use 'static' relocation model for uml modules

[Geert Stappers]

On Tue, Sep 03, 2024 at 09:06:05PM +0800, Zigit Zo wrote:
> In the x86_64 UML, kernel modules compiled with Rust will currently
> generate some R_X86_64_GOTPCREL relocations, which will then be rejected
> by the kernel.
> 
> And because of the kernel modules of UML are only got handled by the UML
> itself, relocation model 'static' can work as expected other than 'pie'.
> ---
> diff --git a/arch/um/Makefile b/arch/um/Makefile
> index 00b63bac5eff..b04b1d4d6dfe 100644
> --- a/arch/um/Makefile
> +++ b/arch/um/Makefile
> @@ -63,7 +63,7 @@ KBUILD_CFLAGS += $(CFLAGS) $(CFLAGS-y) -D__arch_um__ \
>  	-Din6addr_loopback=kernel_in6addr_loopback \
>  	-Din6addr_any=kernel_in6addr_any -Dstrrchr=kernel_strrchr
>  
> -KBUILD_RUSTFLAGS += -Crelocation-model=pie
> +KBUILD_RUSTFLAGS_KERNEL += -Crelocation-model=pie
>  
>  KBUILD_AFLAGS += $(ARCH_INCLUDE)
>  
> -- 

I failed to match the actual change with the commit message.  Consider
that ignorance on my side.

I see a problem in just renaming a variable, might be my problem.  Thing
is that removal of 'KBUILD_RUSTFLAG += ' feels wrong, hence this posting.



Groeten
Geert Stappers
-- 
Silence is hard to parse

Re: [PATCH 1/2] rust: arch/um: use 'static' relocation model for uml modules

[Zigit Zo]

On Wed, Sep 04, 2024 at 11:30:12AM +0200, Geert Stappers wrote:
> I failed to match the actual change with the commit message.  Consider
> that ignorance on my side.

FYI, I'm using the master branch of the upstream, but yes it's in the
'rust-next' or other branches as well, so maybe not a big deal IMO.

Of what 'https://rust-for-linux.com/branches' says, maybe I'll use the
'rust-next' branch for the next time, thanks for pointing out!

> I see a problem in just renaming a variable, might be my problem.  Thing
> is that removal of 'KBUILD_RUSTFLAG += ' feels wrong, hence this posting.

I think this is fine, the 'KBUILD_RUSTFLAG' controls the 'PIE' flag for
both the kernel itself and the loadable modules. And yes, that's the only
way I can make the 'PIE' flag out of the modules while keeping the UML
kernel itself works, and the 'scripts/Makefile.lib' I tought has placed
those flags correctly.

What about adding a 'static' flag to the 'arch/um/Makefile' explicitly,

---
KBUILD_RUSTFLAGS_KERNEL += -Crelocation-model=pie
KBUILD_RUSTFLAGS_MODULE += -Crelocation-model=static
---

but that's just a redundant of the already defined 'KBUILD_RUSTFLAGS'.
Have no more ideas what can this flag goes wrong :(