[lld-macho] Support linking against stub dylibs

Stub dylibs differ from "real" dylibs in that they lack any content in their sections. What they do have are export tries and symbol tables, which means we can still link against them. I am unclear how to properly create these stub dylibs; XCode 11.3's `lipo` is able to create stub dylibs, but those lack LC_ID_DYLIB load commands and are considered invalid by most tooling. Newer versions of `lipo` aren't able to create stub dylibs at all. However, recent SDKs in XCode still come with valid stub dylibs, so it still seems worthwhile to support them. The YAML in this diff's test was generated by taking a non-stub dylib and editing the appropriate fields. Reviewed By: #lld-macho, smeenai Differential Revision: https://reviews.llvm.org/D89012
llvm · Nov 10, 2020 · 2e8e1bd · 2e8e1bd
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diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
@@ -312,6 +312,7 @@ static InputFile *addFile(StringRef path) {
     newFile = make<ObjFile>(mbref);
     break;
   case file_magic::macho_dynamically_linked_shared_lib:
+  case file_magic::macho_dynamically_linked_shared_lib_stub:
     newFile = make<DylibFile>(mbref);
     break;
   case file_magic::tapi_file: {

diff --git a/lld/test/MachO/dylib-stub.yaml b/lld/test/MachO/dylib-stub.yaml
@@ -0,0 +1,138 @@
+## Stub dylibs differ from "real" dylibs in that they lack any content in their
+## sections. What they do have are export tries and symbol tables, which means
+## we can still link against them. I (jezng) am unclear how to properly create
+## these stub dylibs; XCode 11.3's `lipo` is able to create stub dylibs, but
+## those lack LC_ID_DYLIB load commands and are considered invalid by most
+## tooling. Newer versions of `lipo` aren't able to create stub dylibs at all.
+## However, recent SDKs in XCode still come with valid stub dylibs, so it still
+## seems worthwhile to support them. The YAML in this test was generated by
+## taking a non-stub dylib and editing the appropriate fields.
+
+# REQUIRES: x86
+# RUN: split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %t/test.s -o %t/test.o
+# RUN: yaml2obj %t/fat.dylib.yaml > fat.dylib
+# RUN: %lld -lSystem %t/test.o fat.dylib -o %t/test
+# RUN: llvm-objdump --macho --lazy-bind %t/test | FileCheck %s
+# CHECK: __DATA   __la_symbol_ptr    0x100001008 foo  _foo
+
+#--- test.s
+.globl _main
+_main:
+  callq _foo
+  ret
+
+#--- fat.dylib.yaml
+--- !fat-mach-o
+FatHeader:
+  magic:           0xCAFEBABE
+  nfat_arch:       1
+FatArchs:
+  - cputype:         0x01000007
+    cpusubtype:      0x00000003
+    offset:          0x0000000000001000
+    size:            16432
+    align:           12
+Slices:
+  - !mach-o
+    FileHeader:
+      magic:           0xFEEDFACF
+      cputype:         0x01000007
+      cpusubtype:      0x00000003
+      filetype:        0x00000009
+      ncmds:           5
+      sizeofcmds:      568
+      flags:           0x00100085
+      reserved:        0x00000000
+    LoadCommands:
+      - cmd:             LC_SEGMENT_64
+        cmdsize:         232
+        segname:         __TEXT
+        vmaddr:          0
+        vmsize:          16384
+        fileoff:         0
+        filesize:        16384
+        maxprot:         5
+        initprot:        5
+        nsects:          1
+        flags:           0
+        Sections:
+          - sectname:        __text
+            segname:         __TEXT
+            addr:            0x0000000000000000
+            size:            0
+            offset:          0x00003FB7
+            align:           0
+            reloff:          0x00000000
+            nreloc:          0
+            flags:           0x80000400
+            reserved1:       0x00000000
+            reserved2:       0x00000000
+            reserved3:       0x00000000
+            content:         ''
+      - cmd:             LC_SEGMENT_64
+        cmdsize:         72
+        segname:         __LINKEDIT
+        vmaddr:          16384
+        vmsize:          16384
+        fileoff:         16384
+        filesize:        48
+        maxprot:         1
+        initprot:        1
+        nsects:          0
+        flags:           0
+      - cmd:             LC_ID_DYLIB
+        cmdsize:         40
+        dylib:
+          name:            24
+          timestamp:       1
+          current_version: 0
+          compatibility_version: 0
+        PayloadString:   foo.dylib
+        ZeroPadBytes:    7
+      - cmd:             LC_DYLD_INFO_ONLY
+        cmdsize:         48
+        rebase_off:      0
+        rebase_size:     0
+        bind_off:        0
+        bind_size:       0
+        weak_bind_off:   0
+        weak_bind_size:  0
+        lazy_bind_off:   0
+        lazy_bind_size:  0
+        export_off:      16384
+        export_size:     16
+      - cmd:             LC_SYMTAB
+        cmdsize:         24
+        symoff:          16408
+        nsyms:           1
+        stroff:          16424
+        strsize:         8
+    LinkEditData:
+      ExportTrie:
+        TerminalSize:    0
+        NodeOffset:      0
+        Name:            ''
+        Flags:           0x0000000000000000
+        Address:         0x0000000000000000
+        Other:           0x0000000000000000
+        ImportName:      ''
+        Children:
+          - TerminalSize:    3
+            NodeOffset:      8
+            Name:            _foo
+            Flags:           0x0000000000000000
+            Address:         0x0000000000003FB7
+            Other:           0x0000000000000000
+            ImportName:      ''
+      NameList:
+        - n_strx:          2
+          n_type:          0x0F
+          n_sect:          1
+          n_desc:          0
+          n_value:         16311
+      StringTable:
+        - ' '
+        - _foo
+        - ''
+...