Re: An unsigned overflow in luai_makeseed()

On Tue, Feb 17, 2026 at 01:50:39AM -0800, Sergey Bronnikov wrote:
> Hello,
> 
> the function luai_makeseed() in lauxlib.c has the following expression [1]:
> 
> res ^= (res >> 3) + (res << 7) + buff[i];
> 
> where res has type "unsigned integer". This arithmetic has an unsigned 
> overflow.
> This is of course undefined behavior from a language point of view, but 

If res has type "signed int", this is true, ISO 9899:1999 6.5.5 states

> If an exceptional condition occurs during the evaluation of an
> expression (that is, if the result is not mathematically defined or
> not in the range of representable values for its type), the behavior
> is undefined.

However res has type "unsigned int", and for unsigned types we have an
extra rule in 6.2.5.9,

> A computation involving unsigned operands can never overflow, because
> a result that cannot be represented by the resulting unsigned integer
> type is reduced modulo the number that is one greater than the largest
> value that can be represented by the resulting type.

So the behavior is of course defined :)

> maybe it is used here intentionally?

Sure, having the arithmetic result truncated isn't a problem here. We're
making a seed instead of calculating a specific value, what res is in
the end doesn't matter as long as it makes a good use of the entropy.

> 1. https://github.com/lua/lua/blob/c6b484823806e08e1756b1a6066a3ace6f080fae/lauxlib.c#L1167
> 
> Sergey

Best regards,
Yao Zi

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