elf: Detect if Elf is PIE

[Rtoax]

It is not sufficient to determine whether Elf is ET_DYN or not to be a dynamic library in bcc_elf_is_shared_obj() function, because the executable Elf of the PIE type is also ET_DYN.

For example, on fedora 40, /usr/bin/bash is PIE:

    $ readelf -h /usr/bin/bash
    ELF Header:
      ...
      Type:                     DYN (Position-Independent Executable file)
      ...

bcc_elf_is_shared_obj() should not return 'true' for /usr/bin/bash.

This commit add a function bcc_elf_is_pie(), maybe we can provide API in bcc_elf.h.

[yonghong-song]

There is a test failure:

raceback (most recent call last):
  File "/bcc/tests/python/test_usdt3.py", line 146, in test_attach1
    self.assertTrue(self.probe_value_3 != 0)
AssertionError: False is not true

could you take a look?

Searching internet, I found the following link:
https://stackoverflow.com/questions/34519521/why-does-gcc-create-a-shared-object-instead-of-an-executable-binary-according-to/55704865#55704865
which seems validating your implementation.
One thing the above link mentioned is for a shared library, if that shared library is executable, it will be clarified as an executable, e.g. ld.so. But ld.so is a special case, so I guess your implementation should be okay. Not sure why test_usdt3.py failed and whether it is related to your patch or not. It will be great if you can double check.

[Rtoax]

There is a test failure:

raceback (most recent call last):
  File "/bcc/tests/python/test_usdt3.py", line 146, in test_attach1
    self.assertTrue(self.probe_value_3 != 0)
AssertionError: False is not true

could you take a look?

Searching internet, I found the following link: https://stackoverflow.com/questions/34519521/why-does-gcc-create-a-shared-object-instead-of-an-executable-binary-according-to/55704865#55704865 which seems validating your implementation. One thing the above link mentioned is for a shared library, if that shared library is executable, it will be clarified as an executable, e.g. ld.so. But ld.so is a special case, so I guess your implementation should be okay. Not sure why test_usdt3.py failed and whether it is related to your patch or not. It will be great if you can double check.

Thanks for your reply, Song. I think i just found the ISSUE:

In src/cc/usdt/usdt_args.cc::get_global_address() call bcc_elf_is_shared_obj().

bcc uses Ubuntu 20.04 as the CI/CD environment on github. The test ELF program will be compiled as PIE. After that, bcc_elf_is_shared_obj() will skip PIE, causing the py_test_usdt3 test to fail.

Similarly, it will be compiled as PIE on Debian-like systems and as EXEC on Fedora-like systems.

On Fedora 40

      Start 30: py_test_usdt3
30/37 Test #30: py_test_usdt3 ....................   Passed    2.30 sec

On Debian12

    Start 2: py_test_usdt3
30/37 Test #2: py_test_usdt3 ....................***Failed   67.42 sec