[ELF] Reject certain unknown section types (#85173)

Unknown section sections may require special linking rules, and
rejecting such sections for older linkers may be desired. For example,
if we introduce a new section type to replace a control structure (e.g.
relocations), it would be nice for older linkers to reject the new
section type. GNU ld allows certain unknown section types:

* [SHT_LOUSER,SHT_HIUSER] and non-SHF_ALLOC
* [SHT_LOOS,SHT_HIOS] and non-SHF_OS_NONCONFORMING

but reports errors and stops linking for others (unless
--no-warn-mismatch is specified). Port its behavior. For convenience, we
additionally allow all [SHT_LOPROC,SHT_HIPROC] types so that we don't
have to hard code all known types for each processor.

Close #84812

Author: MaskRay

lld/ELF/Config.h

@@ -273,6 +273,7 @@ struct Config {
   bool printGcSections;
   bool printIcfSections;
   bool printMemoryUsage;
+  bool rejectMismatch;
   bool relax;
   bool relaxGP;
   bool relocatable;

lld/ELF/Driver.cpp

@@ -1348,6 +1348,7 @@ static void readConfigs(opt::InputArgList &args) {
   config->printArchiveStats = args.getLastArgValue(OPT_print_archive_stats);
   config->printSymbolOrder =
       args.getLastArgValue(OPT_print_symbol_order);
+  config->rejectMismatch = !args.hasArg(OPT_no_warn_mismatch);
   config->relax = args.hasFlag(OPT_relax, OPT_no_relax, true);
   config->relaxGP = args.hasFlag(OPT_relax_gp, OPT_no_relax_gp, false);
   config->rpath = getRpath(args);

lld/ELF/InputFiles.cpp

@@ -741,6 +741,15 @@ template <class ELFT> void ObjFile<ELFT>::initializeJustSymbols() {
   sections.resize(numELFShdrs);
 }
 
+static bool isKnownSpecificSectionType(uint32_t t, uint32_t flags) {
+  if (SHT_LOUSER <= t && t <= SHT_HIUSER && !(flags & SHF_ALLOC))
+    return true;
+  if (SHT_LOOS <= t && t <= SHT_HIOS && !(flags & SHF_OS_NONCONFORMING))
+    return true;
+  // Allow all processor-specific types. This is different from GNU ld.
+  return SHT_LOPROC <= t && t <= SHT_HIPROC;
+}
+
 template <class ELFT>
 void ObjFile<ELFT>::initializeSections(bool ignoreComdats,
                                        const llvm::object::ELFFile<ELFT> &obj) {
@@ -752,14 +761,15 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats,
     if (this->sections[i] == &InputSection::discarded)
       continue;
     const Elf_Shdr &sec = objSections[i];
+    const uint32_t type = sec.sh_type;
 
     // SHF_EXCLUDE'ed sections are discarded by the linker. However,
     // if -r is given, we'll let the final link discard such sections.
     // This is compatible with GNU.
     if ((sec.sh_flags & SHF_EXCLUDE) && !config->relocatable) {
-      if (sec.sh_type == SHT_LLVM_CALL_GRAPH_PROFILE)
+      if (type == SHT_LLVM_CALL_GRAPH_PROFILE)
         cgProfileSectionIndex = i;
-      if (sec.sh_type == SHT_LLVM_ADDRSIG) {
+      if (type == SHT_LLVM_ADDRSIG) {
         // We ignore the address-significance table if we know that the object
         // file was created by objcopy or ld -r. This is because these tools
         // will reorder the symbols in the symbol table, invalidating the data
@@ -778,7 +788,7 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats,
       continue;
     }
 
-    switch (sec.sh_type) {
+    switch (type) {
     case SHT_GROUP: {
       if (!config->relocatable)
         sections[i] = &InputSection::discarded;
@@ -801,12 +811,25 @@ void ObjFile<ELFT>::initializeSections(bool ignoreComdats,
     case SHT_RELA:
     case SHT_NULL:
       break;
-    case SHT_LLVM_SYMPART:
-      ctx.hasSympart.store(true, std::memory_order_relaxed);
-      [[fallthrough]];
+    case SHT_PROGBITS:
+    case SHT_NOTE:
+    case SHT_NOBITS:
+    case SHT_INIT_ARRAY:
+    case SHT_FINI_ARRAY:
+    case SHT_PREINIT_ARRAY:
+      this->sections[i] =
+          createInputSection(i, sec, check(obj.getSectionName(sec, shstrtab)));
+      break;
     default:
       this->sections[i] =
           createInputSection(i, sec, check(obj.getSectionName(sec, shstrtab)));
+      if (type == SHT_LLVM_SYMPART)
+        ctx.hasSympart.store(true, std::memory_order_relaxed);
+      else if (config->rejectMismatch &&
+               !isKnownSpecificSectionType(type, sec.sh_flags))
+        errorOrWarn(toString(this->sections[i]) + ": unknown section type 0x" +
+                    Twine::utohexstr(type));
+      break;
     }
   }
 

lld/ELF/Options.td

@@ -312,6 +312,9 @@ def no_dynamic_linker: F<"no-dynamic-linker">,
 def noinhibit_exec: F<"noinhibit-exec">,
   HelpText<"Retain the executable output file whenever it is still usable">;
 
+def no_warn_mismatch: F<"no-warn-mismatch">,
+  HelpText<"Suppress errors for certain unknown seciton types">;
+
 def no_nmagic: F<"no-nmagic">, MetaVarName<"<magic>">,
   HelpText<"Page align sections (default)">;
 
@@ -753,7 +756,6 @@ def: FF<"no-add-needed">;
 def: F<"no-copy-dt-needed-entries">;
 def: F<"no-ctors-in-init-array">;
 def: F<"no-keep-memory">;
-def: F<"no-warn-mismatch">;
 def: Separate<["--", "-"], "rpath-link">;
 def: J<"rpath-link=">;
 def: F<"secure-plt">;

lld/docs/ld.lld.1

@@ -378,6 +378,8 @@ Do not put read-only non-executable sections in their own segment.
 Do not report version scripts that refer to undefined symbols.
 .It Fl -no-undefined
 Report unresolved symbols even if the linker is creating a shared library.
+.It Fl -no-warn-mismatch
+Do not reject unknown section types.
 .It Fl -no-warn-symbol-ordering
 Do not warn about problems with the symbol ordering file or call graph profile.
 .It Fl -no-warnings , Fl w

lld/test/ELF/incompatible-section-types2.s

@@ -6,5 +6,5 @@
 // CHECK-NEXT: >>> <internal>:(.shstrtab): SHT_STRTAB
 // CHECK-NEXT: >>> output section .shstrtab: Unknown
 
-.section .shstrtab,"",@12345
+.section .shstrtab,"",@0x60000000
 .short 20

lld/test/ELF/linkerscript/custom-section-type.s

@@ -76,7 +76,7 @@ SECTIONS {
 .section progbits,"a",@note
 .byte 0
 
-.section expr,"a",@12345
+.section expr,"a",@0x60000000
 .byte 0
 
 #--- unknown1.lds

lld/test/ELF/unknown-section.test

@@ -0,0 +1,48 @@
+# RUN: rm -rf %t && mkdir %t && cd %t
+# RUN: yaml2obj %s -o a.o
+# RUN: not ld.lld a.o -o /dev/null 2>&1 | FileCheck %s --implicit-check-not=error:
+
+# CHECK:      error: a.o:(relr): unknown section type 0x13
+# CHECK-NEXT: error: a.o:(regular): unknown section type 0x15
+# CHECK-NEXT: error: a.o:(loos_nonconforming): unknown section type 0x60000000
+# CHECK-NEXT: error: a.o:(hios_nonconforming): unknown section type 0x6fffffff
+# CHECK-NEXT: error: a.o:(louser_alloc): unknown section type 0x80000000
+# CHECK-NEXT: error: a.o:(hiuser_alloc): unknown section type 0xffffffff
+
+--- !ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:  relr
+    Type:  19
+  - Name:  regular
+    Type:  21
+  - Name:  loos
+    Type:  0x60000000
+  - Name:  hios
+    Type:  0x6fffffff
+  - Name:  loos_nonconforming
+    Type:  0x60000000
+    Flags: [ SHF_OS_NONCONFORMING ]
+  - Name:  hios_nonconforming
+    Type:  0x6fffffff
+    Flags: [ SHF_OS_NONCONFORMING ]
+
+  - Name:  loproc
+    Type:  0x70000000
+  - Name:  hiproc
+    Type:  0x7fffffff
+
+  - Name:  louser
+    Type:  0x80000000
+  - Name:  hiuser
+    Type:  0xffffffff
+  - Name:  louser_alloc
+    Type:  0x80000000
+    Flags: [ SHF_ALLOC ]
+  - Name:  hiuser_alloc
+    Type:  0xffffffff
+    Flags: [ SHF_ALLOC ]