CODEWITHSUNDEEP
windowsoperating-systemexecutableexevirtual-address-space
user1232138
user1232138

Reputation: 5541

Need for relocations in an exe

Why is there a need for relocation table when every element in an exe is at a relative offset from the base of the image?? I mean even if the image gets dispacled by a positive offset of say 0X60000, why is there for relocation table, as we would anyways be using RVA's which would be relative to the new base??

Upvotes: 3

Views: 2438

Answers (2)

David Heffernan
David Heffernan

Reputation: 613481

For an EXE file there is no need for the relocation table because the executable image is always loaded at its preferred address. The relocation table can safely be stripped.

Upvotes: 1

valdo
valdo

Reputation: 12951

The point is that the code doesn't access the globals (global variables and function addresses) via RVA or whats-or-ever. They're accessed by their absolute address. And this address should be changed in case the executable was not loaded at its preferred address.

The relocation table consists exactly of those places. It's a table of all the places that should be adjusted by the difference of the actual base address and the preferred one.

BTW, EXEs, in contrast to DLLs usually don't contain relocation tables. This is because they're the first module to be mapped into the address space, hence they may always be loaded at their preferred address. The situation is different for DLLs, which usually do contains relocation tables.

P.S. In Windows 7 EXE may contain relocation table in case they prefer to be loaded at random address. It's a security feature (pitiful IMHO)

Edit:

Should be mentioned that function addresses are not always accessed by their absolute value. On x86 branching instructions (such as jmp, call and etc.) have a "short" format which works with the relative offset. Such places don't neet to be mentioned in relocation table.

Upvotes: 3

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