Accessing String Table Indexes for .rodata

[Henry C]

Hi Mark,

I just dumped out my executable (code was in my very first email):
$ eu-readelf -l myexec

Program Headers:
  Type           Offset   VirtAddr           PhysAddr
FileSiz  MemSiz   Flg Align
  PHDR           0x000040 0x0000000000400040 0x0000000000400040
0x0001f8 0x0001f8 R E 0x8
  INTERP         0x000238 0x0000000000400238 0x0000000000400238
0x00001c 0x00001c R   0x1
        [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x000000 0x0000000000400000 0x0000000000400000
0x00079c 0x00079c R E 0x200000
  LOAD           0x000de8 0x0000000000600de8 0x0000000000600de8
0x000247 0x000248 RW  0x200000
  DYNAMIC        0x000df8 0x0000000000600df8 0x0000000000600df8
0x000200 0x000200 RW  0x8
  NOTE           0x000254 0x0000000000400254 0x0000000000400254
0x000044 0x000044 R   0x4
  GNU_EH_FRAME   0x00063c 0x000000000040063c 0x000000000040063c
0x000044 0x000044 R   0x4
  GNU_STACK      0x000000 0x0000000000000000 0x0000000000000000
0x000000 0x000000 RW  0x10
  GNU_RELRO      0x000de8 0x0000000000600de8 0x0000000000600de8
0x000218 0x000218 R   0x1

 Section to Segment mapping:
  Segment Sections...
   00
   01      [RO: .interp]
   02      [RO: .interp .note.ABI-tag .note.gnu.build-id .gnu.hash
.dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init
.plt .plt.got .text .fini .rodata .eh_frame_hdr .eh_frame]
   03      [RELRO: .init_array .fini_array .dynamic .got] .got.plt .data .bss
   04      [RELRO: .dynamic]
   05      [RO: .note.ABI-tag .note.gnu.build-id]
   06      [RO: .eh_frame_hdr]
   07
   08      [RELRO: .init_array .fini_array .dynamic .got]

By looking at the output and the wiki link, the program headers do
contain virtual addresses but nothing explicit about .rodata.  How can
I use libelf to figure the correct virtual memory address to "hello
world"?

But if I print out Elf64_Shdr::sh_addr (.rodata section header) +
offset to "hello world" in .rodata, I do see the output matches the
output of printf("%p\n", "hello world");

As you mentioned, it is not guaranteed, I wonder under what situation
Elf64_Shdr::sh_addr won't represent the virtual address of the
beginning of .rodata section.

Thanks in advance!
On Fri, Aug 31, 2018 at 7:33 PM Mark Wielaard <mark@klomp.org> wrote:
>
> On Fri, 2018-08-31 at 19:07 +0800, Henry C wrote:
> > Thanks for replying.
> >
> > I mean for example,
> > void myprintf(const char* ptr) {
> >         printf("%p\n", ptr);
> > }
> >
> > int main() {
> >         myprintf("hello world");
> > }
> >
> > Let's say the output is 0x403DE.
> >
> > Does it mean that Elf64_Shdr::sh_addr of the string table (of
> > .rodata)
> > + the offset of "hello word" within the string table is guaranteed to
> > be the virtual address 0x403DE?  As I am not sure whether virtual
> > memory addresses of all string literals are defined/calculated when
> > ELF is created.
>
> No, that is not guaranteed. How data in an ELF file is mapped into
> memory is determined by the program headers (not the section headers).
> See also the picture showing the different ELF file data views in https
> ://en.wikipedia.org/wiki/Executable_and_Linkable_Format
>
> Cheers,
>
> Mark