rcutorture: Fix divide-by-zero with fwd_progress_div=1

When fwd_progress_div=1, the forward-progress test computes:
  sd4 = (sd + div - 1) / div = sd
  dur = sd4 + torture_random(&trs) % (sd - sd4) = sd4 + % 0

The modulo operation with a zero divisor triggers an integer
division by zero (undefined behavior at the C level, #DE trap on x86),
causing a kernel Oops and panic. On x86_64, this manifests as:

  rcu_torture_fwd_prog_nr: Starting forward-progress test 0
  Oops: divide error: 0000 [#1] SMP PTI
  RIP: 0010:rcu_torture_fwd_prog+0x90b/0x1160
  R12: 0000000000000000

The existing guard only handles non-positive values. However,
fwd_progress_div=1 also makes the random range empty because
sd4 == sd.

Change the guard to reject values below 2. The forward-progress test
only reaches this calculation when stall_dur() is positive, so
sd = stall_dur() + 1 >= 2. For fwd_progress_div >= 2, sd4 < sd,
ensuring that sd - sd4 is at least 1.

Keep the existing fallback to the default value of 4 for invalid
values.

Verified with QEMU/KVM: a 138-second run with fwd_progress_div=1
completed 81 forward-progress test cycles without a crash.

Fixes: 1b27291b1ea4f ("rcutorture: Add forward-progress tests for RCU grace periods")
Signed-off-by: Kunwu Chan <kunwu.chan@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

0 files changed, 0 insertions, 0 deletions