[lld-macho] Fix segfault when handling LTO + object file weak defs

[int3]

Summary

which occurs when there are EH frames present in the object file's weak
def.

[abrachet]

Thanks

lld/MachO/InputFiles.cpp

for (const Subsection &subsec : ehFrameSection.subsections) {
     Defined *funcSym;
     if (funcAddrRelocIt != isec->relocs.end()) {
       funcSym = targetSymFromCanonicalSubtractor(isec, funcAddrRelocIt);
       // Canonicalize the symbol. If there are multiple symbols at the same
       // address, we want both `registerEhFrame` and `registerCompactUnwind`
       // to register the unwind entry under same symbol.
       // This is not particularly efficient, but we should run into this case
       // infrequently (only when handling the output of `ld -r`).
+      if (funcSym->isec)
         funcSym = findSymbolAtOffset(cast<ConcatInputSection>(funcSym->isec),
                                      funcSym->value);
     } else {
       funcSym = findSymbolAtAddress(sections, funcAddr);
       ehRelocator.makePcRel(funcAddrOff, funcSym, target->p2WordSize);
     }
     // The symbol has been coalesced, or already has a compact unwind entry.
     if (!funcSym || funcSym->getFile() != this || funcSym->unwindEntry) {
       // We must prune unused FDEs for correctness, so we cannot rely on
       // -dead_strip being enabled.

lld/test/MachO/Inputs/lto-obj-weak-def.yaml

+--- !mach-o
+FileHeader:
+  magic:           0xFEEDFACF
+  cputype:         0x1000007
+  cpusubtype:      0x3
+  filetype:        0x1
+  ncmds:           3
+  sizeofcmds:      352
+  flags:           0x0
+  reserved:        0x0
+LoadCommands:
+  - cmd:             LC_SEGMENT_64
+    cmdsize:         312
+    segname:         ''
+    vmaddr:          0
+    vmsize:          96
+    fileoff:         416
+    filesize:        96
+    maxprot:         7
+    initprot:        7
+    nsects:          3
+    flags:           0
+    Sections:
+      - sectname:        __text
+        segname:         __TEXT
+        addr:            0x0
+        size:            1
+        offset:          0x1A0
+        align:           0
+        reloff:          0x0
+        nreloc:          0
+        flags:           0x80000400
+        reserved1:       0x0
+        reserved2:       0x0
+        reserved3:       0x0
+        content:         C3
+      - sectname:        __eh_frame
+        segname:         __TEXT
+        addr:            0x8
+        size:            56
+        offset:          0x1A8
+        align:           3
+        reloff:          0x200
+        nreloc:          4
+        flags:           0x0
+        reserved1:       0x0
+        reserved2:       0x0
+        reserved3:       0x0
+        content:         1400000000000000017A520001781001100C0708900100001C00000004000000F8FFFFFFFFFFFFFF0100000000000000000E080000000000
+        relocations:
+          - address:         0x1C
+            symbolnum:       0
+            pcrel:           false
+            length:          2
+            extern:          true
+            type:            5
+            scattered:       false
+            value:           0
+          - address:         0x1C
+            symbolnum:       1
+            pcrel:           false
+            length:          2
+            extern:          true
+            type:            0
+            scattered:       false
+            value:           0
+          - address:         0x20
+            symbolnum:       1
+            pcrel:           false
+            length:          3
+            extern:          true
+            type:            5
+            scattered:       false
+            value:           0
+          - address:         0x20
+            symbolnum:       2
+            pcrel:           false
+            length:          3
+            extern:          true
+            type:            0
+            scattered:       false
+            value:           0
+      - sectname:        __compact_unwind
+        segname:         __LD
+        addr:            0x40
+        size:            32
+        offset:          0x1E0
+        align:           3
+        reloff:          0x220
+        nreloc:          1
+        flags:           0x2000000
+        reserved1:       0x0
+        reserved2:       0x0
+        reserved3:       0x0
+        content:         '0000000000000000010000000000000400000000000000000000000000000000'
+        relocations:
+          - address:         0x0
+            symbolnum:       2
+            pcrel:           false
+            length:          3
+            extern:          true
+            type:            0
+            scattered:       false
+            value:           0
+  - cmd:             LC_SYMTAB
+    cmdsize:         24
+    symoff:          552
+    nsyms:           3
+    stroff:          600
+    strsize:         32
+  - cmd:             LC_DATA_IN_CODE
+    cmdsize:         16
+    dataoff:         552
+    datasize:        0
+LinkEditData:
+  NameList:
+    - n_strx:          7
+      n_type:          0xE
+      n_sect:          2
+      n_desc:          0
+      n_value:         8
+    - n_strx:          17
+      n_type:          0xE
+      n_sect:          2
+      n_desc:          0
+      n_value:         32
+    - n_strx:          2
+      n_type:          0xF
+      n_sect:          1
+      n_desc:          160
+      n_value:         0
+  StringTable:
+    - ' '
+    - _foo
+    - EH_Frame1
+    - func.eh
+    - ''
+    - ''
+    - ''
+    - ''
+    - ''
+    - ''
+    - ''
+...

lld/test/MachO/lto-obj-weak-def.ll

+; REQUIRES: x86
+
+;; Verify that we successfully merge weak definitions across bitcode and regular
+;; assembly files, even when EH frames are present. We would previously
+;; segfault.
+
+; RUN: rm -rf %t; split-file %s %t
+; RUN: llvm-as %t/foo-1.ll -o %t/foo-1.o
+
+;; When changing the assembly input, uncomment these lines to re-generate the
+;; YAML.
+; COM: llvm-mc --emit-dwarf-unwind=always -filetype=obj -triple=x86_64-apple-darwin %t/foo-2.s -o %t/foo-2.o
+; COM: ld -r %t/foo-2.o -o %t/foo-2-r.o
+; COM: obj2yaml %t/foo-2-r.o -o %S/Inputs/lto-obj-weak-def.yaml
+
+; RUN: yaml2obj %S/Inputs/lto-obj-weak-def.yaml -o %t/foo-2-r.o 
+; RUN: %lld -lSystem -dylib %t/foo-1.o %t/foo-2-r.o -o /dev/null
+
+;--- foo-1.ll
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define weak void @foo() {
+  ret void
+}
+
+;--- foo-2.s
+.globl _foo
+.weak_definition _foo
+_foo:
+  .cfi_startproc
+  .cfi_def_cfa_offset 8
+  ret
+  .cfi_endproc