CODEWITHSUNDEEP
clinux
Einheri
Einheri

Reputation: 985

Questions about loading executables into memory

So I'm working with the linux 0.11 kernel on a virtual machine, and I need to write a program that analyses executable files that are ran on that kernel. The files are in the format a.out. What I want to know is, how does the operating system decide where to load the file in (virtual?) memory? Is it decided by something called "base address", and if so, how come I can't seem to find any mention of it in the a.out header?

//where is base address?
struct exec {
  unsigned long a_magic;    /* Use macros N_MAGIC, etc for access */
  unsigned a_text;          /* length of text, in bytes */
  unsigned a_data;          /* length of data, in bytes */
  unsigned a_bss;           /* length of uninitialized data area for file, in bytes */
  unsigned a_syms;          /* length of symbol table data in file, in bytes */
  unsigned a_entry;         /* start address */
  unsigned a_trsize;        /* length of relocation info for text, in bytes */
  unsigned a_drsize;        /* length of relocation info for data, in bytes */
};

I tried looking for documentations about the format, but the only information I found just explains what each of these fields are, what values a_magic can have, etc.

I need to know about it because the program needs to print out file and line numbers when given an address in memory of an instruction in the executable, and the debug symbols only have their addresses as offsets (e.g. relative to the start of text section, etc).

Also, out of curiosity, I know that in C, "(void*)0" is NULL, which you can't dereference. How then would you get the content of memory address 0?

As you see, I know very little about linux kernel and operating systems in general, so please start from the basics...

I appreciate any help you can give, thanks.

Upvotes: 0

Views: 151

Answers (3)

florin
florin

Reputation: 14326

Also, out of curiosity, I know that in C, "(void*)0" is NULL, which you can't dereference. How then would you get the content of memory address 0?

Actually you can dereference NULL, but the results are not defined. For convenience, most operating systems trap the access to help you debug pointer problems.

Also, memory location with address 0 in a process space is different from the memory location with address 0 in the 'hardware space'. The 'pagination' support in the CPU and operating system are 'decoupling' the physical memory from the virtual memory. It could happen that a virtual page be mapped at address 0, although there you usually have interrupt vectors and other special device memory and not real RAM anyway.

Upvotes: 0

Pekka
Pekka

Reputation: 3654

The operating system can load the application at any location it chooses and then relocate the embedded addresses to be relative to that point. This relocation information is recorded in the a.out file. The base address depends on the architecture and other details and is often non-zero.

If you look at a linker map file, you should see a symbol that is either at the beginning of the memory image, or at a fixed offset from it. At runtime, subtract this value from the actual addresses you note for debugging to get to the relative address of the position you are interested in.

Note, the symbols will not be present in the executable if your linker script strips them.

Upvotes: 0

David Schwartz
David Schwartz

Reputation: 182763

The base address is the a_entry field.

Also, out of curiosity, I know that in C, "(void*)0" is NULL, which you can't dereference. How then would you get the content of memory address 0?

Any system that puts memory usable by a C program at address zero would have to make it work, somehow. While one can imagine possible ways to do this, I don't know of anyone who bothers. Virtual address zero is, for all intents and purposes, never used.

Upvotes: 1

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