RE: [PATCH] net: stmmac: Fix signed/unsigned wreckage

From: Thomas Gleixner

Sent: 15 November 2021 15:21

The recent addition of timestamp correction to compensate the CDC error
introduced a subtle signed/unsigned bug in stmmac_get_tx_hwtstamp() while
it managed for some obscure reason to avoid that in stmmac_get_rx_hwtstamp().

The issue is:

    s64 adjust = 0;
    u64 ns;

    adjust += -(2 * (NSEC_PER_SEC / priv->plat->clk_ptp_rate));
    ns += adjust;

works by chance on 64bit, but falls apart on 32bit because the compiler
knows that adjust fits into 32bit and then treats the addition as a u64 +
u32 resulting in an off by ~2 seconds failure.

The problem is earlier.
NSEC_PER_SEC and clk_ptp_rate are almost certainly 32bit and unsigned.
So you have:
	adjust = (s64)((u64)adjust + (u32)-(2 * (NSEC_PER_SEC/...)));

	David

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