autoPatchelfHook skips position-independent executables

[tadfisher]

Issue description

autoPatchelfHook determines which executables it should patch via:

isExecutable() {
    readelf -h "$1" 2> /dev/null | grep -q '^ *Type: *EXEC\>'
}

However, PIEs have a type of DYN, as in:

$ readelf -h result/build-tools/28.0.3/zipalign
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            UNIX - System V
  ABI Version:                       0
  Type:                              DYN (Shared object file)
  Machine:                           Advanced Micro Devices X86-64
  Version:                           0x1
  Entry point address:               0x5350
  Start of program headers:          241664 (bytes into file)
  Start of section headers:          198080 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         10
  Size of section headers:           64 (bytes)
  Number of section headers:         32
  Section header string table index: 27

Questions:

• Should autoPatchelfHook also patch PIEs? An alternative is to just call autoPatchelfFile manually, but I'm having trouble finding out how to run it in the correct phase.
• Is there even a reliable way to distinguish between shared libraries and PIEs? I know in general, shared libs don't have an .interp section, but is this a reliable enough assumption?

cc @aszlig

[aszlig]

@tadfisher: Actually, executables can also be shared libraries at the same time (eg. libc.so.6 and I'm doing exactly that in my recent project as well), so the only thing that distinguishes them is an .interp section (except if statically linked of course). So I guess it's a better idea to just look for .interp in isExecutable instead of what we currently do. At least if I don't miss something, this should be the most reliable way, after all what we want to do with executables is to patch the .interp section.

If nobody else sends a PR in the meantime, I'm going to fix this next week.

As for autoPatchelfFile it's a bit more difficult, because it needs to be run after the normal patchelf hook, so it might be necessary to add postPhases.