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gccelfreadelf

VirtualAddress, LoadAddress, and PhysicalAddress in ELF file?

According to the ld manual on Output Section Description:

 section [address] [(type)] :
   [AT(lma)]
   [ALIGN(section_align) | ALIGN_WITH_INPUT]
   [SUBALIGN(subsection_align)]
   [constraint]
   {
     output-section-command
     output-section-command
     ...
   } [>region] [AT>lma_region] [:phdr :phdr ...] [=fillexp] [,]

The address or >region stand for the VMA, i.e. the Virtual Memory Address of the output section.

The AT() or AT>lma_region stand for the LMA, i.e. the Load Memory Address of the output section.

And I decide get a close view with readelf -e to dump the section headers and program headers of a helloworld elf file. The result is below:

section headers

program headers

My questions are:

  • Why there's no LMA in the dumped headers? How is LMA represented in ELF file?
  • What does the Addr column in the red rectangle mean? VMA?
  • What does the PhysAddr in the green rectangle mean?

ADD 1

So far, It seems the PhysAddr is the LMA.

Solution

  • Why there's no LMA in the dumped headers? How is the LMA represented in an ELF file

    Firstly there is no LMA header within an elf file, it is actually quiet simple, multiple sections in an ELF file are mapped into segments, if the sections mapped into segments have a LOAD flag for example (PROFBITS) is a loadable section type, and the segment they are mapped into is also a load type segment (INTERP and LOAD) for example are also loadable segments, that means every section within that segment within that elf file would be loaded into memory. where? simply to the VMA they were given, so no there is no LMA in an elf file, a LMA is represented by a VMA given that the section should be loaded which is a specified type / flag.

    What does the addr column in the red rectangle mean?

    This has a direct correlation to your previous question, Yes! it does mean a VMA, in order to have this properly explained we need to understand that an ELF format was designed for architectures that support some memory protection / memory segmentation. you might want to give some section special permissions, instead of giving every section it's own memory protection, you'll map multiple sections into a segment and give that sole segment it's own memory protections.

    This causes the need to map sections into segment, how would the OS loader know how to map each section into segment and by that give it the appropriate memory protection? by it's address. Each section is also given an address and by those addresses / offsets / sizes they are mapped into a segment which in overall would be allocated into memory and given some memory protection rules that would apply to all sections.

    The only way that the OS could know how to map these is by address so yes if the section is of a loadable type it's ADDR means VMA ( at least for modern systems that use Virtual Memory and dont abuse the elf file )

    What does the PhysAddr mean?

    As much as I know, PhysAddr is only relevant to old fashioned architectures in which physical addressing is relevant to user-space programs, this section should hold the actual physical address the segment would sit in, yet in most modern systems this is simply ignored...

    I suggest you read this http://flint.cs.yale.edu/cs422/doc/ELF_Format.pdf,

    personally back in the day when learning this, it helped me a lot and gave me a lot of knowledge regarding ELF files hopefully I've helped you some how! :)