On Thu Sep 10, 2026 at 11:08 AM BST, Mukesh Kumar Chaurasiya (IBM) wrote:
> The Rust kernel infrastructure generates inline asm for WARN() via
> ARCH_WARN_ASM(file, line, flags, size), expanding it through a C
> preprocessor pass (generated_arch_warn_asm.rs.S) to produce an
> arch-specific asm template string for use in Rust's core::arch macros.
>
> powerpc currently lacks ARCH_WARN_ASM and ARCH_WARN_REACHABLE, causing
> Rust builds to fail on powerpc with
> ```
> error: no rules expected `ARCH_WARN_ASM`
> -->
> /home/linkmauve/dev/linux/wii/rust/kernel/generated_arch_warn_asm.rs:1:28
> |
> 1 | ::kernel::concat_literals!(ARCH_WARN_ASM("{file}", "{line}", "{flags}",
> "{size}"))
I think we probably want to catch this earlier by have something like
#ifndef ARCH_WARN_ASM
#error "ARCH_WARM_ASM is not defined"
#endif
in generated_arch_warn_asm.rs.S.
> | ^^^^^^^^^^^^^ no rules expected this token
> in macro call
> |
> ::: ../rust/kernel/lib.rs:279:1
> |
> 279 | macro_rules! concat_literals {
> | ---------------------------- when calling this macro
> |
> = note: while trying to match sequence start
>
> error: no rules expected `ARCH_WARN_REACHABLE`
> -->
> /home/linkmauve/dev/linux/wii/rust/kernel/generated_arch_reachable_asm.rs:1:28
> |
> 1 | ::kernel::concat_literals!(ARCH_WARN_REACHABLE)
> | ^^^^^^^^^^^^^^^^^^^ no rules expected this
> token in macro call
> |
> ::: ../rust/kernel/lib.rs:279:1
> |
> 279 | macro_rules! concat_literals {
> | ---------------------------- when calling this macro
> |
> = note: while trying to match sequence start
>
> error: aborting due to 2 previous errors
> ```
>
> To add ARCH_WARN_ASM, _EMIT_BUG_ENTRY first needs to be refactored.
> The old definition was a bare macro with no parameters, relying on
> positional asm operand references (%0-%3), hardcoding the backward
> reference to local label 1b, and including .org/.previous directives
> inline. That made it impossible to compose as a plain string outside of
> an asm operand context, and left an invisible contract that callers must
> always emit their trap at label 1:.
>
> Refactor _EMIT_BUG_ENTRY to take explicit (label, file, line, flags)
> string arguments via string concatenation. This removes the dependency
> on asm operand numbering and makes the trap label an explicit argument,
> so the caller's intent is visible at the call site and a future caller
> using a different label cannot silently produce a wrong bug table entry.
>
> Move the .org and .previous directives out of _EMIT_BUG_ENTRY and into
> each call site, so BUG_ENTRY() can still pass sizeof(struct bug_entry)
> as an asm operand while ARCH_WARN_ASM can supply its own size string
> independently.
>
> Add ARCH_WARN_REACHABLE as an empty define, matching the arm64
> convention, indicating that no additional reachability annotation is
> needed after a WARN on powerpc.
>
> This brings powerpc into line with x86, arm64, s390, and riscv, all of
> which already define ARCH_WARN_ASM and ARCH_WARN_REACHABLE.
>
> Reported-by: FUJITA Tomonori <[email protected]>
> Closes: https://lore.kernel.org/all/anG67Q6Y59kDqh-c@desktop
> Fixes: 73b741adb264 ("rust: Add PowerPC support")
> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <[email protected]>
> ---
> Changelog:
> V2 -> V3:
> - Add label argument in _EMIT_BUG_ENTRY
> V2: https://lore.kernel.org/all/[email protected]
>
> V1 -> V2:
> - commit message now has error, fixes tag and closes tag
> V1: https://lore.kernel.org/all/[email protected]
>
> arch/powerpc/include/asm/bug.h | 36 +++++++++++++++++++---------------
> 1 file changed, 20 insertions(+), 16 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> index 0db48977c70c..df2183c35945 100644
> --- a/arch/powerpc/include/asm/bug.h
> +++ b/arch/powerpc/include/asm/bug.h
> @@ -32,34 +32,38 @@
> #endif /* verbose */
>
> #else /* !__ASSEMBLER__ */
> -/* _EMIT_BUG_ENTRY expects args %0,%1,%2,%3 to be FILE, LINE, flags and
> - sizeof(struct bug_entry), respectively */
> #ifdef CONFIG_DEBUG_BUGVERBOSE
> -#define _EMIT_BUG_ENTRY \
> - ".section __bug_table,\"aw\"\n" \
> - "2: .4byte 1b - .\n" \
> - " .4byte %0 - .\n" \
> - " .short %1, %2\n" \
> - ".org 2b+%3\n" \
> - ".previous\n"
> +#define _EMIT_BUG_ENTRY(label, file, line, flags) \
> + ".section __bug_table,\"aw\"\n" \
> + "2: .4byte " label "b - .\n" \
"b" is part of the label. "1b" itself is a label and "1" is just an integer.
If the code uses
_EMIT_BUG_ENTRY(..)
"1: ..."
then the correct label would be "1f".
Best,
Gary
> + " .4byte " file " - .\n" \
> + " .short " line ", " flags "\n"
> #else
> -#define _EMIT_BUG_ENTRY \
> - ".section __bug_table,\"aw\"\n" \
> - "2: .4byte 1b - .\n" \
> - " .short %2\n" \
> - ".org 2b+%3\n" \
> - ".previous\n"
> +#define _EMIT_BUG_ENTRY(label, file, line, flags) \
> + ".section __bug_table,\"aw\"\n" \
> + "2: .4byte " label "b - .\n" \
> + " .short " flags "\n"
> #endif
>
> #define BUG_ENTRY(cond_str, insn, flags, ...) \
> __asm__ __volatile__( \
> "1: " insn "\n" \
> - _EMIT_BUG_ENTRY \
> + _EMIT_BUG_ENTRY("1", "%0", "%1", "%2") \
> + ".org 2b+%3\n" \
> + ".previous\n" \
> : : "i" (WARN_CONDITION_STR(cond_str) __FILE__), "i"
> (__LINE__), \
> "i" (flags), \
> "i" (sizeof(struct bug_entry)), \
> ##__VA_ARGS__)
>
> +#define ARCH_WARN_ASM(file, line, flags, size) \
> + "1: twi 31, 0, 0\n" \
> + _EMIT_BUG_ENTRY("1", file, line, flags) \
> + ".org 2b+" size "\n" \
> + ".previous\n"
> +
> +#define ARCH_WARN_REACHABLE
> +
> /*
> * BUG_ON() and WARN_ON() do their best to cooperate with compile-time
> * optimisations. However depending on the complexity of the condition
