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Negating or taking absolute values of maximally-negative OverflowSafeInt may result in undefined behaviour #8284

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techgeeknz opened this issue Jul 29, 2020 · 1 comment

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@techgeeknz
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@techgeeknz techgeeknz commented Jul 29, 2020

As mentioned in #7924, OverflowSafeInt can produce incorrect results when the value underflows. This appears to be the result of undefined behaviour caused by taking the inverse or absolute value of T_MIN; the magnitude of which cannot usually be represented as a positive value of the same integer type.

I am not yet familiar with OpenTTD's continuous integration and regression testing system; however, I have an idea to work around the issue using typecasts to unsigned integers where appropriate.

techgeeknz added a commit to techgeeknz/OpenTTD that referenced this issue Jul 30, 2020
Rewrite to avoid taking the absolute value of a maximally-negative
integer. Fixes OpenTTD#8284
techgeeknz added a commit to techgeeknz/OpenTTD that referenced this issue Jul 30, 2020
Rewrite to avoid taking the absolute value of a maximally-negative
integer. Fixes OpenTTD#8284
@James103
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@James103 James103 commented Aug 5, 2020

This can also lead to the following exploit, which was used to produce test cases for #8253 and JGRennison/OpenTTD-patches#164:

  1. Start a new game with any loan interest rate. For the fastest reproduction, set your loan interest rate to 4%.
  2. Max out your loan (ctrl-click "Borrow 10,000").
  3. Wait and fast forward for ~800 in-game years.
  4. Your money should now be equal (or very close to) INT64_MAX.

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techgeeknz added a commit to techgeeknz/OpenTTD that referenced this issue Aug 23, 2020
Rewrite to avoid taking the absolute value of a maximally-negative
integer. Fixes OpenTTD#8284
techgeeknz added a commit to techgeeknz/OpenTTD that referenced this issue Aug 31, 2020
Rewrite to avoid taking the absolute value of a maximally-negative
integer. Fixes OpenTTD#8284
JGRennison added a commit to JGRennison/OpenTTD-patches that referenced this issue Oct 2, 2020
JGRennison added a commit to JGRennison/OpenTTD-patches that referenced this issue Oct 2, 2020
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