How to tell whether the p_vaddr in ELF program header is the real memory address or is just an offset from the shared library base address?

Question

I'm trying to locate where the libc program segments are in a program's memory, by reading the program headers.

On Centos 6, when I use readelf on the libc.so.6 file, the VirtAddr contains the correct address where the program segment is loaded in the process's memory:

[user@centos6 src]$ readelf -l /lib64/libc.so.6 --wide

Elf file type is DYN (Shared object file)
Entry point 0x3032c1ee30
There are 10 program headers, starting at offset 64

Program Headers:
  Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
  PHDR           0x000040 0x0000003032c00040 0x0000003032c00040 0x000230 0x000230 R E 0x8
  INTERP         0x15aab0 0x0000003032d5aab0 0x0000003032d5aab0 0x00001c 0x00001c R   0x10
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x000000 0x0000003032c00000 0x0000003032c00000 0x18a00c 0x18a00c R E 0x200000
  LOAD           0x18a700 0x0000003032f8a700 0x0000003032f8a700 0x004f58 0x009228 RW  0x200000
  DYNAMIC        0x18db40 0x0000003032f8db40 0x0000003032f8db40 0x0001f0 0x0001f0 RW  0x8
  NOTE           0x000270 0x0000003032c00270 0x0000003032c00270 0x000044 0x000044 R   0x4
  TLS            0x18a700 0x0000003032f8a700 0x0000003032f8a700 0x000010 0x000068 R   0x8
  GNU_EH_FRAME   0x15aacc 0x0000003032d5aacc 0x0000003032d5aacc 0x0065ec 0x0065ec R   0x4
  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x8
  GNU_RELRO      0x18a700 0x0000003032f8a700 0x0000003032f8a700 0x003900 0x003900 R   0x1

So in this case, the DYNAMIC segment is located at 0x0000003032f8db40

But on Centos 7, the VirtAddr contains an offset instead, and I have to add the libc base address to this offset to find where the segment is in memory:

[user@centos7 src]$ readelf -l /usr/lib64/libc.so.6 --wide

Elf file type is DYN (Shared object file)
Entry point 0x22660
There are 10 program headers, starting at offset 64

Program Headers:
  Type           Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
  PHDR           0x000040 0x0000000000000040 0x0000000000000040 0x000230 0x000230 R E 0x8
  INTERP         0x18eb00 0x000000000018eb00 0x000000000018eb00 0x00001c 0x00001c R   0x10
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x000000 0x0000000000000000 0x0000000000000000 0x1c3170 0x1c3170 R E 0x200000
  LOAD           0x1c36f0 0x00000000003c36f0 0x00000000003c36f0 0x0051b0 0x009b10 RW  0x200000
  DYNAMIC        0x1c6b60 0x00000000003c6b60 0x00000000003c6b60 0x0001f0 0x0001f0 RW  0x8
  NOTE           0x000270 0x0000000000000270 0x0000000000000270 0x000044 0x000044 R   0x4
  TLS            0x1c36f0 0x00000000003c36f0 0x00000000003c36f0 0x000010 0x0000a0 R   0x10
  GNU_EH_FRAME   0x18eb1c 0x000000000018eb1c 0x000000000018eb1c 0x006aec 0x006aec R   0x4
  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000 0x000000 0x000000 RW  0x10
  GNU_RELRO      0x1c36f0 0x00000000003c36f0 0x00000000003c36f0 0x003910 0x003910 R   0x1

In this case, the DYNAMIC segment would be located at 0x00000000003c6b60 + libc base address.

I'm guessing this is probably due to ASLR on Centos 7, which causes the libc library to be loaded at a different address each time. On Centos 6, it seems like the libc is always loaded at the same address.

Is there a way to determine, just from reading the ELF header, whether I need to add the libc base address to the VirtAddr to get the actual location of the program segment in memory?

Answer 1

I think I figured it out, you need to check all of the PT_LOAD segments and find the one with the lowest p_vaddr (I'll call it lowest_pt_load).

Then to work out the memory location it is: libc_base + segment.p_addr - lowest_pt_load.p_vaddr

What was happening in the Centos 6 case was that lowest_pt_load was equal to libc base address, causing them to cancel out.

Source: https://docs.oracle.com/cd/E19683-01/816-1386/6m7qcoblk/index.html#chapter6-83432

Base Address

Executable and shared object files have a base address, which is the lowest virtual address associated with the memory image of the program's object file. One use of the base address is to relocate the memory image of the program during dynamic linking.

An executable or shared object file's base address is calculated during execution from three values: the memory load address, the maximum page size, and the lowest virtual address of a program's loadable segment. The virtual addresses in the program headers might not represent the actual virtual addresses of the program's memory image. See "Program Loading (Processor-Specific)".

To compute the base address, you determine the memory address associated with the lowest p_vaddr value for a PT_LOAD segment. You then obtain the base address by truncating the memory address to the nearest multiple of the maximum page size. Depending on the kind of file being loaded into memory, the memory address might not match the p_vaddr values.