Re: [RFC PATCH v2 5/6] kcov: add interrupt context guard to kcov_df_write()

[Peter Zijlstra]

On Wed, Jun 03, 2026 at 07:43:32PM +0200, Yunseong Kim wrote:
> The KCOV-Dataflow write path (kcov_df_write) only checks
> t->kcov_df_enabled before writing to the shared ring buffer. Unlike
> the standard KCOV check_kcov_mode() which rejects interrupt context,
> kcov_df_write() has no such protection. This means instrumented code
> running in hardirq, softirq, or NMI context that interrupts a task
> mid-write can re-enter kcov_df_write(), causing:
> 
>  - Data corruption in the ring buffer (interleaved records)
>  - Out-of-order sequence counter increments
>  - Potential faults from nested pointer dereferences
> 
> Add an in_task() check to reject calls from non-task context, matching
> the safety model of the standard KCOV tracing path.
> 
> Also suppress -Wmissing-prototypes in the eight_args_c test module
> Makefile, as the exported test functions intentionally lack a shared
> header.
> 
> Signed-off-by: Yunseong Kim <yunseong.kim@xxxxxxxx>
> ---
>  kernel/kcov.c                             | 4 ++++
>  tools/kcov-dataflow/eight_args_c/Makefile | 1 +
>  2 files changed, 5 insertions(+)
> 
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index d3c9c0efe961..373b8034ca5c 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -409,6 +409,10 @@ kcov_df_write(u64 type_marker, u64 pc, u64 meta, void *ptr,
>  	if (!t->kcov_df_enabled)
>  		return;
>  
> +	/* Reject calls from hardirq/softirq/NMI to prevent reentrant corruption. */
> +	if (!in_task())
> +		return;
> +
>  	area = (u64 *)t->kcov_df_area;
>  	if (!area)
>  		return;
> diff --git a/tools/kcov-dataflow/eight_args_c/Makefile b/tools/kcov-dataflow/eight_args_c/Makefile
> index de35bb541f07..038775b49435 100644
> --- a/tools/kcov-dataflow/eight_args_c/Makefile
> +++ b/tools/kcov-dataflow/eight_args_c/Makefile
> @@ -1,2 +1,3 @@
>  obj-m := eight_args_mod.o
>  KCOV_DATAFLOW_eight_args_mod.o := y
> +ccflags-y += -Wno-missing-prototypes

This is a weird commit and probably should not exist. You introduce
kcov_df_write() a few patches ago, why doesn't it add these few lines
there?

Similarly, you introduce this tools thing a few patches ago, fix the
Makefile there?