Re: [PATCH v2 net] ptp: netc: fix period truncation and potential divide-by-zero in PEROUT

On 26-08-21 11:24:49, wei.fang@oss.nxp.com wrote:

From: Wei Fang <wei.fang@nxp.com>

The max_period bound in net_timer_enable_perout() was computed as:

  max_period = (u64)NETC_TMR_DEFAULT_FIPER + integral_period;

which exceeds U32_MAX when integral_period > 0 (e.g. 0x100000002 for
the default 333333333 Hz clock). A period_ns that passes this check but
exceeds U32_MAX is then silently truncated when stored into the u32
struct netc_pp::period field.

A truncated value of zero can reach netc_timer_set_perout_alarm(), where
the local u32 period variable would also be 0, causing a divide-by-zero
in roundup_u64(delta, period) whenever the stime < min_time branch is
taken (which always happens for a start time of {0, 0}).

Additionally, netc_timer_enable_periodic_pulse() and
netc_timer_enable_fiper() both compute:

  fiper = pp->period - integral_period;

A zero pp->period results in an unsigned wraparound to 0xFFFFFFFD,
mis-programming the FIPER hardware register.

Fix all three issues by capping max_period at NETC_TMR_DEFAULT_FIPER
(0xFFFFFFFF). This ensures that any period_ns passing the range check
fits in a u32 without truncation, so the stored value is always valid
and non-zero. The accepted range is reduced by integral_period ns
(typically only a few nanoseconds), which is negligible in practice.

Fixes: 671e266835b8 ("ptp: netc: add periodic pulse output support")
Signed-off-by: Wei Fang <wei.fang@nxp.com>

Reviewed-by: Abel Vesa <redacted>