[COFF] Don't write long section names for sections that will be mapped at runtime

Sections that will be mapped at runtime will only have the short
section name available, since the string table it points into isn't
mapped. Therefore prefer truncating those names over writing a
long name that is unavailable at runtime.

This allows libunwind to find the .eh_frame section at runtime even
if the module was built with debug info enabled.

Differential Revision: https://reviews.llvm.org/D40025

llvm-svn: 318391

lld/COFF/Writer.cpp

@@ -211,7 +211,8 @@ void OutputSection::writeHeaderTo(uint8_t *Buf) {
     // If name is too long, write offset into the string table as a name.
     sprintf(Hdr->Name, "/%d", StringTableOff);
   } else {
-    assert(!Config->Debug || Name.size() <= COFF::NameSize);
+    assert(!Config->Debug || Name.size() <= COFF::NameSize ||
+           (Hdr->Characteristics & IMAGE_SCN_MEM_DISCARDABLE) == 0);
     strncpy(Hdr->Name, Name.data(),
             std::min(Name.size(), (size_t)COFF::NameSize));
   }
@@ -541,6 +542,13 @@ void Writer::createSymbolAndStringTable() {
     StringRef Name = Sec->getName();
     if (Name.size() <= COFF::NameSize)
       continue;
+    // If a section isn't discardable (i.e. will be mapped at runtime),
+    // prefer a truncated section name over a long section name in
+    // the string table that is unavailable at runtime. This is different from
+    // what link.exe does, but finding ".eh_fram" instead of "/4" is useful
+    // to libunwind.
+    if ((Sec->getPermissions() & IMAGE_SCN_MEM_DISCARDABLE) == 0)
+      continue;
     Sec->setStringTableOff(addEntryToStringTable(Name));
   }
 

lld/test/COFF/long-section-name.test

@@ -2,6 +2,7 @@
 # RUN: lld-link /debug /out:%t.exe /entry:main %t.obj
 # RUN: llvm-readobj -sections %t.exe | FileCheck %s
 
+# CHECK: Name: .eh_fram (
 # CHECK: Name: .data_long_section_name
 # CHECK: Name: .text_long_section_name
 
@@ -11,10 +12,14 @@ header:
   Characteristics: [  ]
 sections:
   - Name:            .text_long_section_name
-    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_DISCARDABLE ]
     Alignment:       4
     SectionData:     B82A000000C3
   - Name:            .data_long_section_name
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE, IMAGE_SCN_MEM_DISCARDABLE ]
+    Alignment:       4
+    SectionData:     "00"
+  - Name:            .eh_frame
     Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_READ, IMAGE_SCN_MEM_WRITE ]
     Alignment:       4
     SectionData:     "00"
@@ -49,6 +54,18 @@ symbols:
       NumberOfLinenumbers: 0
       CheckSum:        0
       Number:          0
+  - Name:            .eh_frame
+    Value:           0
+    SectionNumber:   3
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          0
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
   - Name:            main
     Value:           0
     SectionNumber:   1