tools/elf2efi: skip empty .got section and its .relro_padding

Resolves systemd/systemd#31637.

lld-18 does the section setup differently than older versions. There is a bunch
of ordering chagnes, but it also inserts the following:

Sections:
Idx Name          Size      VMA               LMA               File off  Algn
...
  9 .got          00000000  00000000000283c0  00000000000283c0  000283c0  2**3
                  CONTENTS, ALLOC, LOAD, DATA
 10 .relro_padding 00000c40  00000000000283c0  00000000000283c0  000283c0  2**0
                  ALLOC
 11 .data         00000024  00000000000293c0  00000000000293c0  000283c0  2**4
                  CONTENTS, ALLOC, LOAD, DATA
...

This causes a problem for us, because we try to map the .got to .rodata,
and the subsequent .data to .data, and round down the VMA to the nearest
page, which causes the PE sections to overlap.

llvm/llvm-project#66042 adds .relro_padding to make
sure that the RELRO segment is properly write protected and allocated. For our
binaries, the .got section is empty, so we can skip it safely, and the
.relro_padding section is not useful once .got has been dropped.

We don't expect .got sections, but they are apparently inserted on i386 and
aarch64 builds. Emit a warning until we figure out why they are there.

tools/elf2efi.py

@@ -26,6 +26,7 @@
 import io
 import os
 import pathlib
+import sys
 import time
 import typing
 from ctypes import (
@@ -212,6 +213,7 @@ def __init__(self):
     ".eh_frame",
     ".eh_frame_hdr",
     ".ARM.exidx",
+    ".relro_padding",
 ]
 
 IGNORE_SECTION_TYPES = [
@@ -274,10 +276,14 @@ def iter_copy_sections(elf: ELFFile) -> typing.Iterator[PeSection]:
             elf_s["sh_flags"] & SH_FLAGS.SHF_ALLOC == 0
             or elf_s["sh_type"] in IGNORE_SECTION_TYPES
             or elf_s.name in IGNORE_SECTIONS
+            or elf_s["sh_size"] == 0
         ):
             continue
         if elf_s["sh_type"] not in ["SHT_PROGBITS", "SHT_NOBITS"]:
             raise BadSectionError(f"Unknown section {elf_s.name} with type {elf_s['sh_type']}")
+        if elf_s.name == '.got':
+            # FIXME: figure out why those sections are inserted
+            print("WARNING: Non-empty .got section", file=sys.stderr)
 
         if elf_s["sh_flags"] & SH_FLAGS.SHF_EXECINSTR:
             rwx = PE_CHARACTERISTICS_RX