In this tutorial, we will study the file header portion of the PE header.
Let's summarize what we have learned so far:
We will learn more about the PE header in this tutorial. The official name of the PE header is IMAGE_NT_HEADERS. To refresh your memory, I show it below.
Signature dd ?
FileHeader IMAGE_FILE_HEADER <>
OptionalHeader IMAGE_OPTIONAL_HEADER32 <>
Signature is the PE signature, "PE" followed by two zeroes. You already know and use this member.
FileHeader is a structure that contains the information about the physical layout/properies of the PE file in general.
OptionalHeader is also a structure that contains the information about the logical layout inside the PE file.
The most interesting information is in OptionalHeader. However, some fields in FileHeader are also important. We will learn about FileHeader in this tutorial so we can move to study OptionalHeader in the next tutorials.
Machine WORD ?
NumberOfSections WORD ?
TimeDateStamp dd ?
PointerToSymbolTable dd ?
NumberOfSymbols dd ?
SizeOfOptionalHeader WORD ?
Characteristics WORD ?
In summary, only three members are somewhat useful to us: Machine, NumberOfSections and Characteristics. You would normally not change the values of Machine and Characteristics but you must use the value in NumberOfSections when you're walking the section table.
I'm jumping the gun here but in order to illustrate the use of NumberOfSections, I need to digress briefly to the section table.
The section table is an array of structures. Each structure contains the information of a section. Thus if there are 3 sections, there will be 3 members in this array. You need the value in NumberOfSections so you know how many members there are in the array. You would think that checking for the structure with all zeroes in its members would help. Windows does use this approach. You can verify this fact by setting the value in NumberOfSections to a value higher than the real value and Windows still runs the file without problem. From my observation, I think Windows reads the value in NumberOfSections and examines each structure in the section table. If it finds a structure that contains all zeroes, it teRminates the search. Else it would process until the number of structures specified in NumberOfSections is met. Why can't we ignore the value in NumberOfSections? Several reasons. The PE specification doesn't specify that the section table array must end with an all-zero structure. Thus there may be a situation where the last array member is contiguous to the first section, without empty space at all. Another reason has to do with bound imports. The new-style binding puts the information immediately following the section table's last structure array member. Thus you still need NumberOfSections.
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