Re: [PATCH] tracing: Disable KCOV instrumentation for trace_irqsoff.o

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Mon, 25 May 2026 19:04:28 +0200
Karl Mehltretter <kmehltretter@gmail.com> wrote:

> When KCOV runs its boot selftest with whole-kernel instrumentation
> enabled, it sets current->kcov_mode to KCOV_MODE_TRACE_PC without
> installing a coverage area. Any instrumented code accepted as task-context
> coverage in that window dereferences current->kcov_area and crashes.
> 
> On ARMv5 Versatile PB with CONFIG_KCOV_SELFTEST=y,
> CONFIG_KCOV_INSTRUMENT_ALL=y and CONFIG_IRQSOFF_TRACER=y, boot hits a
> NULL pointer fault during the selftest:
> 
>   kcov: running self test
>   Internal error: Oops: 5 [#1] ARM
>   PC is at __sanitizer_cov_trace_pc+0x4c/0x90
>   Kernel panic - not syncing: Fatal exception
> 
> A diagnostic run showed the unwanted coverage comes from the IRQs-off
> tracer callbacks reached from ARM IRQ entry before hardirq context is
> visible to KCOV:
> 
>   __sanitizer_cov_trace_pc from tracer_hardirqs_off+0x18/0x1cc
>   tracer_hardirqs_off from trace_hardirqs_off+0x34/0x54
>   trace_hardirqs_off from __irq_svc+0x58/0xb0
>   __irq_svc from kcov_init+0x7c/0xdc
> 
> and similarly through tracer_hardirqs_on().
> 
> trace_preemptirq.o is already excluded because this tracing path can run
> from early interrupt code and produce coverage unrelated to syscall
> inputs. Exclude trace_irqsoff.o as well, instead of requiring users to
> turn off CONFIG_KCOV_INSTRUMENT_ALL=y, which is the default whole-kernel
> KCOV mode.
> 
> With the exclusion in place, the same ARMv5 Versatile PB QEMU test boots
> through the KCOV selftest and reaches userspace.
> 
> Tested on ARMv5 Versatile PB QEMU with CONFIG_KCOV_SELFTEST=y,
> CONFIG_KCOV_INSTRUMENT_ALL=y and CONFIG_IRQSOFF_TRACER=y.


Thanks for reporting. This looks good to me for a mitigation.
BTW, I could not reproduce the bug with above configs.
Is this only for arm32?


> 
> Assisted-by: Codex:gpt-5
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
> ---
>  kernel/trace/Makefile | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
> index 8d3d96e847d8..f934ff586bd4 100644
> --- a/kernel/trace/Makefile
> +++ b/kernel/trace/Makefile
> @@ -48,9 +48,10 @@ ifdef CONFIG_GCOV_PROFILE_FTRACE
>  GCOV_PROFILE := y
>  endif
>  
> -# Functions in this file could be invoked from early interrupt
> -# code and produce random code coverage.
> +# Functions in these files can run from IRQ entry before hardirq context
> +# is visible to KCOV, and produce coverage unrelated to syscall inputs.
>  KCOV_INSTRUMENT_trace_preemptirq.o := n
> +KCOV_INSTRUMENT_trace_irqsoff.o := n
>  
>  CFLAGS_bpf_trace.o := -I$(src)
>  
> -- 
> 2.39.5 (Apple Git-154)
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>