kcov: report spurious PCs in the interrupt selftest

The KCOV interrupt selftest enables KCOV_MODE_TRACE_PC without a coverage
area so that spurious coverage causes a fault.  If the fault path is
instrumented, the coverage callback faults recursively.  Observed failure
modes include a stack overflow on x86_64 and arm32 getting stuck in abort
handling, neither of which identifies the original coverage event.

The recursive failure is not new, but commit 9a79524d1420 ("kcov: use
WRITE_ONCE() for selftest mode stores") made the selftest effective on
configurations where the compiler had previously removed the mode store,
exposing it more broadly.

Replace the open-coded kcov_mode stores with kcov_start() and kcov_stop(),
passing a 16-word buffer that records up to 15 spurious PCs, with the
first word holding the count.  Keep KCOV enabled for the original 300 ms
window, then disable it, print all recorded PCs and panic if any were
recorded.  Once the buffer fills, further PCs are discarded by the
existing coverage callback.  This preserves the selftest's hard failure
while avoiding the recursive fault path.

Add decanonicalize_ip() as the inverse of canonicalize_ip(), which
subtracts kaslr_offset() from recorded PCs.  Restore that offset before
printing the PCs with %pB, since KCOV records return addresses.

Link: https://lore.kernel.org/20261002181357.14293-1-kmehltretter@gmail.com
Fixes: 6cd0dd934b03 ("kcov: Add interrupt handling self test")
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Tested-by: Bradley Morgan <brads@mainlining.org> # Power10
Reviewed-by: Bradley Morgan <brads@mainlining.org>
Assisted-by: LLM
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Bradley Morgan <include@grrlz.net>
Cc: Thomas Gleixner <tglx@kernel.org>

0 files changed, 0 insertions, 0 deletions