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Frankie say relax

Peter Ferrie
Virus Bulletin, August 2011, page 4-6
ISSN 0956-9979
August 2011

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When a virus infects a file, it usually needs to know its loading address so that it can access its variables. This is done most commonly by using a ‘delta offset’. There are two main types of delta offset: one is the difference between the location where the virus is currently loaded and the original location where the virus was loaded when it was created; the other is the difference between the location of the variable and the start of the virus code. One alternative method is to append relocation items to the host relocation table (if one exists), so that the addresses in the virus code are updated appropriately by the operating system itself. However, touching the host relocation table can be a complex task, depending on the file format and its location within the file. Another alternative is to carry a relocation table in the virus body and use that to update the addresses to constant values during the infection phase. This is the method that is used by Linux/Relax.A. Linux/Relax.B uses the same method, but in this case the relocation table is generated dynamically.

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