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

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Message-ID: <f1f2c082-0597-4130-91d2-2059df3ba72f@intel.com>
Date: Wed, 5 Nov 2025 15:47:00 -0800
From: Jacob Keller <jacob.e.keller@...el.com>
To: Nate Karstens <nate.karstens@...min.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@...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



On 11/5/2025 3:12 PM, Nate Karstens wrote:
> 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)
> 

Ah, right so if we don't cast skb->len then the entire thing uses
unsigned arithmetic which results in the bad outcome for certain values
of input.

Casting would fix this. Another alternative would be to re-write the
checks so that they don't fail when using unsigned arithmetic.

Given that we already cast one to ssize_t, it does seem reasonable to
just add the other cast as your patch did.
>> 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.
> 

Right. Fixing it makes the most sense, so that other users don't
accidentally behave unexpectedly.

> Cheers,
> 
> Nate


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