Re: [PATCH] kcov: Use WRITE_ONCE() for selftest mode stores

[Alexander Potapenko]

On Tue, May 26, 2026 at 1:47 PM Karl Mehltretter <kmehltretter@xxxxxxxxx> wrote:
>
> The KCOV selftest enables coverage by setting current->kcov_mode to
> KCOV_MODE_TRACE_PC without installing a coverage area. If an interrupt
> records coverage in that window, the access should fault and expose the
> bug.
>
> When building for QEMU raspi0 (Raspberry Pi Zero, ARMv6,
> CONFIG_CPU_V6K=y, CONFIG_CURRENT_POINTER_IN_TPIDRURO=y) with GCC
> 13.3.0, the store that enables the mode is removed. The generated
> kcov_init() code only stores zero after the wait loop:
>
>   mrc   15, 0, r3, cr13, cr0, {3}
>   str   r4, [r3, #2028]
>
> where r4 is zero. There is no store of KCOV_MODE_TRACE_PC before the
> loop, so the selftest reports success without exercising coverage.
>
> Use WRITE_ONCE() for the temporary mode stores. With the same compiler
> and config, kcov_init() contains the intended mode store:
>
>   mov   r3, #2
>   mrc   15, 0, r2, cr13, cr0, {3}
>   str   r3, [r2, #2028]
>
> Now that the KCOV selftest is actually executed, it may expose KCOV
> instrumentation issues depending on the kernel config. That is expected
> for a selftest that was intended to catch coverage from interrupt paths.
>
> Fixes: 6cd0dd934b03 ("kcov: Add interrupt handling self test")
> Assisted-by: Codex:gpt-5
> Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
Reviewed-by: Alexander Potapenko <glider@xxxxxxxxxx>