Re: [PATCH V5 2/2] rust: kernel: Add KUnit tests for powerpc ARCH_WARN_ASM bug table emission

[FUJITA Tomonori <tomo@flapping.org>]

On Tue, 15 Sep 2026 14:34:53 +0530
"Mukesh Kumar Chaurasiya (IBM)" <mkchauras@gmail.com> wrote:

> Verify that the __bug_table entry emitted by ARCH_WARN_ASM has a correct
> bug_addr displacement ― i.e. the '1b' label reference in _EMIT_BUG_ENTRY
> resolves to the trap instruction ― by calling find_bug() with the exact
> virtual address of the twi instruction, mirroring what the real powerpc
> trap handler does.
> 
> The trap address is captured at link time via a .dc.a 1b relocation placed
> in .data by the global_asm! block. global_asm! is used instead of asm!
> because LLVM eliminates asm! blocks in dead branches; global_asm! is
> file-scope and always emitted. BUG_KUNIT_TRAP_ADDR is defined as a .global
> symbol directly on the .dc.a word so the linker relocation lands on it ―
> a Rust static initialized to zero would end up in BSS where relocations are
> not applied.
> 
> .dc.a emits a pointer-width word (4 bytes on ppc32, 8 bytes on ppc64),
> so BUG_KUNIT_TRAP_ADDR is declared as usize on the Rust side, making the
> tests correct on both ppc32 and ppc64. The global_asm! block is split into
> two cfg-gated variants (CONFIG_PPC64 / !CONFIG_PPC64) to select the right
> .balign since concat!() only accepts literals.
> 
> Five tests are included in the rust_kernel_bug_powerpc suite:
> 
>   bug_entry_found         - find_bug() returns non-NULL for the trap address,
>                             proving the bug_addr displacement is correct
>   bug_entry_is_warning    - the emitted entry has BUGFLAG_WARNING set
>   bug_entry_file          - bug_get_file_line() returns the correct source
>                             file (requires CONFIG_DEBUG_BUGVERBOSE)
>   bug_entry_line          - the recorded line number is non-zero, confirming
>                             the {line} operand was substituted correctly
>                             (requires CONFIG_DEBUG_BUGVERBOSE)
>   bug_entry_addr_is_in_text - kernel_text_address() confirms the trap address
>                             lies in kernel text, not data or zero
> 
> The suite is named rust_kernel_bug_powerpc and the Kconfig option
> CONFIG_RUST_BUG_POWERPC_KUNIT_TEST depends on PPC && GENERIC_BUG,
> covering both ppc32 and ppc64.
> 
> Tested on ppc64le (ltcfujiaac-lp3, 7.3.0-rc1+): pass:5 fail:0 skip:0.
> Tested on ppc32 Book3S (QEMU mac99 G4, chrp32_defconfig): pass:5 fail:0 skip:0.
> Tested on ppc64le (QEMU pseries, pseries_le_defconfig): pass:5 fail:0 skip:0.
> 
> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> ---
>  rust/kernel/Kconfig.test |  13 +++
>  rust/kernel/bug.rs       | 209 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 222 insertions(+)

I think that testing warn_on!, in particular that the right address is
put in the bug entry, is a good idea.

I'd prefer such a test to work on every architecture. This one depends on
details of powerpc's ARCH_WARN_ASM, such as the '1b' label. KUnit has a
warning suppression API. A test can call warn_on! inside a suppression
block and check that one warning was counted. The counter is only
incremented after find_bug() found the entry, so a wrong address is caught
too.

I'll send a patch for that shortly.