[llvm-objcopy] Add --only-keep-debug as a noop

[jakehehrlich]

Summary

This option just keeps being a problem and really needs to be implemented in some fashion. Implementing it properly requires some kind of "replaceSectionReference" method because all the existing links need to be maintained. The desired behavior is just for allocated sections to become NOBITS but actually implementing that is rather tricky due to the current design of llvm-objcopy. However converting allocated sections to NOBITS is just an optimization and not something debuggers need. Debuggers can debug a stripped executable and take an unstripped executable for that stripped executable as input. Additionally allocated sections account for a very small part of debug binaries so this optimization is quite small. I propose that for the time being we implement this as a NOP so that people can use llvm-objcopy where they need to, just in a sub-optimal way.

This option has already blocked a lot of people and its currently blocking me.

[phosek]

LGTM

[jhenderson]

From what I understand of what you are saying, this switch isn't really that critical to users, but it's needed for command-line compatibility. Is that right?

LGTM, if so.

[jakehehrlich]

I forgot to tag this. Closing.

llvm/test/tools/llvm-objcopy/basic-only-keep-debug.test

+# NOTE: This test is only intended to be valid as long as --only-keep-debug is
+#       implemented as a NOP. This test should fail when that changes and you
+#       will need to update this test.
+
+# RUN: yaml2obj %s > %t
+# RUN: llvm-objcopy %t %t2
+# RUN: llvm-objcopy --only-keep-debug %t %t3
+# RUN: cmp %t2 %t3
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_EXEC
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    Content:         "DEADBEEF"

llvm/tools/llvm-objcopy/ObjcopyOpts.td

def discard_all : Flag<["-", "--"], "discard-all">,
                       HelpText<"Remove all local symbols except file and section symbols">;
 def x : Flag<["-"], "x">,
         Alias<discard_all>;
 defm strip_symbol : Eq<"strip-symbol">,
                        MetaVarName<"symbol">,
                        HelpText<"Remove symbol <symbol>">;
 def N : JoinedOrSeparate<["-"], "N">,
         Alias<strip_symbol>;
+def only_keep_debug : Flag<["-", "--"], "only-keep-debug">,
+                          HelpText<"Currently ignored. Only for compaitability with GNU objcopy.">;

llvm/tools/llvm-objcopy/llvm-objcopy.cpp

struct CopyConfig {
   bool StripDebug = false;
   bool StripSections = false;
   bool StripNonAlloc = false;
   bool StripDWO = false;
   bool ExtractDWO = false;
   bool LocalizeHidden = false;
   bool Weaken = false;
   bool DiscardAll = false;
+  bool OnlyKeepDebug = false;
 };
 
 using SectionPred = std::function<bool(const SectionBase &Sec)>;
 
 bool IsDWOSection(const SectionBase &Sec) { return Sec.Name.endswith(".dwo"); }
 
 bool OnlyKeepDWOPred(const Object &Obj, const SectionBase &Sec) {
   // We can't remove the section header string table.
CopyConfig ParseObjcopyOptions(ArrayRef<const char *> ArgsArr) {
   Config.StripDebug = InputArgs.hasArg(OBJCOPY_strip_debug);
   Config.StripDWO = InputArgs.hasArg(OBJCOPY_strip_dwo);
   Config.StripSections = InputArgs.hasArg(OBJCOPY_strip_sections);
   Config.StripNonAlloc = InputArgs.hasArg(OBJCOPY_strip_non_alloc);
   Config.ExtractDWO = InputArgs.hasArg(OBJCOPY_extract_dwo);
   Config.LocalizeHidden = InputArgs.hasArg(OBJCOPY_localize_hidden);
   Config.Weaken = InputArgs.hasArg(OBJCOPY_weaken);
   Config.DiscardAll = InputArgs.hasArg(OBJCOPY_discard_all);
+  Config.OnlyKeepDebug = InputArgs.hasArg(OBJCOPY_only_keep_debug);
   for (auto Arg : InputArgs.filtered(OBJCOPY_localize_symbol))
     Config.SymbolsToLocalize.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_globalize_symbol))
     Config.SymbolsToGlobalize.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_weaken_symbol))
     Config.SymbolsToWeaken.push_back(Arg->getValue());
   for (auto Arg : InputArgs.filtered(OBJCOPY_strip_symbol))
     Config.SymbolsToRemove.push_back(Arg->getValue());