Re: [PATCH V13 5/7] rust: Make __udivdi3() and __umoddi3() panic

Link Mauve
2026-04-24T09:43:20Z

On Fri, Apr 24, 2026 at 10:14:25AM +0100, David Laight wrote:
> On Fri, 24 Apr 2026 11:17:40 +0530
> "Mukesh Kumar Chaurasiya (IBM)" <mkchauras@gmail.com> wrote:
>
> > From: Link Mauve <linkmauve@linkmauve.fr>
> >
> > The core crate currently depends on these two functions for i64/u64/
> > i128/u128/core::time::Duration formatting, but we shouldn’t use that in
> > the kernel so let’s panic if they are ever called.
>
> Ugg.
> Surely you can make it a link-time failure?
> Or change the underlying code to avoid the divide.

This is currently a link-time failure, which means if any of the core
crate or drm_panic_qr are linked in, the kernel will fail to link.

And since we rely on core, that means no kernel can be built on PowerPC
without this patch.

A possible solution would be to change the core crate to not divide
u64s, but the last time I tried to do that I couldn’t quite figure out
how to do formatting without it, maybe I will just open an issue against
Rust.

>
> David
>
> >
> > This doesn’t yet fix drm_panic_qr.rs, which also uses __udivdi3 when
> > CONFIG_CC_OPTIMIZE_FOR_SIZE=y, but at least makes the rest of the kernel
> > build on PPC32.
> >
> > Signed-off-by: Link Mauve <linkmauve@linkmauve.fr>
> > Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> > ---
> > rust/Makefile | 4 ++++
> > rust/compiler_builtins.rs | 6 ++++++
> > 2 files changed, 10 insertions(+)
> >
> > diff --git a/rust/Makefile b/rust/Makefile
> > index 2a5428a5503d..ee11fba7a03d 100644
> > --- a/rust/Makefile
> > +++ b/rust/Makefile
> > @@ -616,6 +616,10 @@ ifneq ($(or $(CONFIG_ARM64),$(and $(CONFIG_RISCV),$(CONFIG_64BIT))),)
> > __ashrti3 \
> > __ashlti3 __lshrti3
> > endif
> > +ifdef CONFIG_PPC32
> > + redirect-intrinsics += \
> > + __udivdi3 __umoddi3
> > +endif
> >
> > ifdef CONFIG_MODVERSIONS
> > cmd_gendwarfksyms = $(if $(skip_gendwarfksyms),, \
> > diff --git a/rust/compiler_builtins.rs b/rust/compiler_builtins.rs
> > index dd16c1dc899c..fc6b54636dd5 100644
> > --- a/rust/compiler_builtins.rs
> > +++ b/rust/compiler_builtins.rs
> > @@ -97,5 +97,11 @@ pub extern "C" fn $ident() {
> > __aeabi_uldivmod,
> > });
> >
> > +#[cfg(target_arch = "powerpc")]
> > +define_panicking_intrinsics!("`u64` division/modulo should not be used", {
> > + __udivdi3,
> > + __umoddi3,
> > +});
> > +
> > // NOTE: if you are adding a new intrinsic here, you should also add it to
> > // `redirect-intrinsics` in `rust/Makefile`.
>
>

--
Link Mauve