Reland D61583 [ELF] Error on relocations to STT_SECTION symbols if the sections were discarded

This restores r361830 "[ELF] Error on relocations to STT_SECTION symbols if the sections were discarded"
and dependent commits (r362218, r362497) which were reverted by r364321, with a fix of a --gdb-index issue.

.rela.debug_ranges contains relocations of range list entries:

    // start address of a range list entry
    // old: 0; after r361830: 0
    00000000000033a0 R_X86_64_64 .text._ZN2v88internal7Isolate7factoryEv + 0
    // end address of a range list entry
    // old: 0xe; after r361830: 0
    00000000000033a8 R_X86_64_64 .text._ZN2v88internal7Isolate7factoryEv + e

If both start and end addresses of a range list entry resolve to 0,
DWARFDebugRangeList::isEndOfListEntry() will return true, then the
.debug_range decoding loop will terminate prematurely:

    while (true) {
      decode StartAddress
      decode EndAddress
      if (Entry.isEndOfListEntry()) // prematurely
        break;
      Entries.push_back(Entry);
    }

In lld/ELF/SyntheticSections.cpp, readAddressAreas() will read
incomplete address ranges and the resulting .gdb_index will be
incomplete. For files that gdb hasn't loaded their debug info, gdb uses
.gdb_index to map addresses to CUs. The absent entries make gdb fail to
symbolize some addresses.

To address this issue, we simply allow relocations to undefined symbols
in DWARF.cpp:findAux() and let RelocationResolver resolve them.

This patch should fix:

[1] http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190603/659848.html
[2] https://bugs.chromium.org/p/chromium/issues/detail?id=978067

llvm-svn: 364391

1 files changed, 51 insertions, 0 deletions

diff --git a/lld/test/ELF/gdb-index-ranges-discarded.s b/lld/test/ELF/gdb-index-ranges-discarded.s
new file mode 100644
index 00000000000..2d5ce15736f
--- /dev/null
+++ b/lld/test/ELF/gdb-index-ranges-discarded.s
@@ -0,0 +1,51 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld --gdb-index -e main %t.o -o %t
+# RUN: llvm-dwarfdump -gdb-index %t | FileCheck %s
+
+# CHECK:      .gdb_index contents:
+# CHECK:       Address area offset = 0x28, has 1 entries:
+# CHECK-NEXT:    Low/High address = [0x201000, 0x201001) (Size: 0x1), CU id = 0
+
+## .debug_ranges contains 2 entries. .Lfunc_end0 is defined in the discarded
+## .text.foo. Test we resolve it to a non-zero value, otherwise the address area
+## of .gdb_index will not included [.Lfunc_begin1, .Lfunc_end1).
+
+.section .text.foo,"aex",@progbits
+.Lfunc_begin0:
+  ret
+.Lfunc_end0:
+
+.section .text.bar,"ax",@progbits
+.Lfunc_begin1:
+  ret
+.Lfunc_end1:
+
+.section .debug_abbrev,"",@progbits
+  .byte 1         # Abbreviation Code
+  .byte 17        # DW_TAG_compile_unit
+  .byte 0         # DW_CHILDREN_no
+  .byte 85        # DW_AT_ranges
+  .byte 23        # DW_FORM_sec_offset
+  .byte 0         # EOM(1)
+  .byte 0         # EOM(2)
+  .byte 0
+
+.section .debug_info,"",@progbits
+.Lcu_begin0:
+  .long .Lcu_end0 - .Lcu_begin0 - 4
+  .short 4        # DWARF version number
+  .long  0        # Offset Into Abbrev. Section
+  .byte  8        # Address Size
+.Ldie0:
+  .byte  1        # Abbrev [1] DW_TAG_compile_unit
+  .long  0        # DW_AT_ranges
+.Lcu_end0:
+
+.section .debug_ranges,"",@progbits
+  .quad .Lfunc_begin0
+  .quad .Lfunc_end0
+  .quad .Lfunc_begin1
+  .quad .Lfunc_end1
+  .quad 0
+  .quad 0