linux-kernel - Re: [PATCH] strparser: Fix signed/unsigned mismatch bug

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Message-ID: <20251105231212.1491817-1-nate.karstens@garmin.com>
Date: Wed, 5 Nov 2025 17:12:12 -0600
From: Nate Karstens <nate.karstens@...min.com>
To: <jacob.e.keller@...el.com>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <horms@...nel.org>,
        <john.fastabend@...il.com>, <kuba@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux@...blig.org>, <mrpre@....com>,
        <nate.karstens@...min.com>, <nate.karstens@...il.com>,
        <netdev@...r.kernel.org>, <pabeni@...hat.com>,
        <stable@...r.kernel.org>, <tom@...ntonium.net>
Subject: Re: [PATCH] strparser: Fix signed/unsigned mismatch bug

Thanks, Jake!

> So, without the ssize_t, I guess everything switches back to unsigned
> here when subtracting skb->len..

That's right. In C, if there is a mix of signed an unsigned, then signed are converted to unsigned and unsigned arithmetic is used.

> I don't quite recall the signed vs unsigned rules for this. Is
> stm.strp.offset also unsigned? which means that after head->len -
> skb->len resolves to unsigned 0 then we underflow?

Here is a summary of the types for the variables involved:

len => ssize_t (signed)
(ssize_t)head->len => unsigned int cast to ssize_t
skb->len => unsigned int, causes the whole comparison to use unsigned arithmetic
stm->strp.offset => int (see struct strp_msg)

> If we don't actually use the strparser code anywhere then it could be
> dropped

It is still used elsewhere, and ktls still uses some of the data structures.

Cheers,

Nate

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