CODEWITHSUNDEEP
llvmexeelfdisassembly
codeman48
codeman48

Reputation: 1430

Increase size of the BSS section in the elf and other file formats

I want to add a 4KB space to the bss section of an executable elf file. How can this be done?

If not bss section, can I increase the size of the data section or define a new data section and initialize the region ?

If possible, can this be done with binary file formats other than elf ?

PS: I need this to add some extra instructions for instrumentation and saving the data in dedicated locations which have to be printed later.

Upvotes: 1

Views: 1101

Answers (1)

Employed Russian
Employed Russian

Reputation: 213646

I want to add a 4KB space to the bss section of an executable elf file. How can this be done?

Assuming you want to do that to an already linked ELF executable, note that sections are not used at all by anything (other than perhaps debugging tools) after the link is done; you want to modify the corresponding segment.

What you are asking for is impossible in general, but might be possible for your particular executable. In particular, it should be possible if the LOAD segment "covering" the .bss section is the last one, or if there is an in-memory gap between that segment and the next LOAD segment.

If there is space (in memory) to extend the LOAD segment in question, then all you have to do is patch its program header's .p_memsz and increment it by 4096.

You would need to understand the output from readelf -Wl a.out in detail.

Update:

assuming that bss occurs last, is there a tool to change .p_memsz of the last segment in a line or two ?

I don't know of any tool to do this, but it's pretty trivial: program headers are fixed sized table starting at file offset ehdr->e_phoff. The table contains ehdr->e_phnum records. You read each record until you find the last PT_LOAD segment, update the .p_memsz member, seek back and write the updated record on top of what was there.

The libelf or elfio libraries may (or may not) make writing this tool easier.

I guess to make the elf conformant, we also need to change the section size of bss accordingly ?

No, you don't:

  • there is no requirement that sizes of .bss and the load segment match
  • in fact, there is no requirement that any sections are present at all,
  • like I said, nothing cares about any sections after the binary is linked.

Upvotes: 3

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