On Mon, 25 May 2026 19:04:28 +0200 Karl Mehltretter <[email protected]> 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 <[email protected]> > --- > 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) <[email protected]>
