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

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Message-ID: <50343373035f4f2694aac4df86dae618@AcuMS.aculab.com>
Date:   Tue, 16 Nov 2021 13:06:49 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Thomas Gleixner' <tglx@...utronix.de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "David S. Miller" <davem@...emloft.net>,
        Voon Weifeng <weifeng.voon@...el.com>,
        Wong Vee Khee <vee.khee.wong@...el.com>,
        "Giuseppe Cavallaro" <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...s.st.com>,
        Jose Abreu <joabreu@...opsys.com>,
        "Benedikt Spranger" <b.spranger@...utronix.de>
Subject: 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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