[llvm-objcopy] preserve file ownership when overwritten by root

As of binutils 2.36, GNU strip calls chown(2) for "sudo strip foo" and
"sudo strip foo -o foo", but no "sudo strip foo -o bar" or "sudo strip
foo -o ./foo". In other words, while "sudo strip foo -o bar" creates a
new file bar with root access, "sudo strip foo" will keep the owner and
group of foo unchanged. Currently llvm-objcopy and llvm-strip behave
differently, always changing the owner and gropu to root. The
discrepancy prevents Chrome OS from migrating to llvm-objcopy and
llvm-strip as they change file ownership and cause intended users/groups
to lose access when invoked by sudo with the following sequence
(recommended in man page of GNU strip).

1.<Link the executable as normal.>
1.<Copy "foo" to "foo.full">
1.<Run "strip --strip-debug foo">
1.<Run "objcopy --add-gnu-debuglink=foo.full foo">

This patch makes llvm-objcopy and llvm-strip follow GNU's behavior.

Link: crbug.com/1108880

3 files changed, 15 insertions, 2 deletions

diff --git a/llvm/tools/llvm-objcopy/Buffer.cpp b/llvm/tools/llvm-objcopy/Buffer.cpp
index 06b2a20..3049794 100644
--- a/llvm/tools/llvm-objcopy/Buffer.cpp
+++ b/llvm/tools/llvm-objcopy/Buffer.cpp
@@ -36,7 +36,12 @@ Error FileBuffer::allocate(size_t Size) {
   }
 
   Expected<std::unique_ptr<FileOutputBuffer>> BufferOrErr =
-      FileOutputBuffer::create(getName(), Size, FileOutputBuffer::F_executable);
+      FileOutputBuffer::create(getName(), Size,
+                               KeepOwnership
+                                   ? FileOutputBuffer::F_executable |
+                                         FileOutputBuffer::F_keep_ownership
+                                   : FileOutputBuffer::F_executable,
+                               UserID, GroupID);
   // FileOutputBuffer::create() returns an Error that is just a wrapper around
   // std::error_code. Wrap it in FileError to include the actual filename.
   if (!BufferOrErr)
diff --git a/llvm/tools/llvm-objcopy/Buffer.h b/llvm/tools/llvm-objcopy/Buffer.h
index 487d558..e439e98 100644
--- a/llvm/tools/llvm-objcopy/Buffer.h
+++ b/llvm/tools/llvm-objcopy/Buffer.h
@@ -40,6 +40,9 @@ class FileBuffer : public Buffer {
   // Indicates that allocate(0) was called, and commit() should create or
   // truncate a file instead of using a FileOutputBuffer.
   bool EmptyFile = false;
+  bool KeepOwnership = false;
+  unsigned UserID = 0;
+  unsigned GroupID = 0;
 
 public:
   Error allocate(size_t Size) override;
@@ -47,6 +50,8 @@ public:
   Error commit() override;
 
   explicit FileBuffer(StringRef FileName) : Buffer(FileName) {}
+  explicit FileBuffer(StringRef FileName, bool Keep, unsigned UID, unsigned GID)
+      : Buffer(FileName), KeepOwnership(Keep), UserID(UID), GroupID(GID) {}
 };
 
 class MemBuffer : public Buffer {
diff --git a/llvm/tools/llvm-objcopy/llvm-objcopy.cpp b/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
index 7fd2acd..42d97b2 100644
--- a/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
+++ b/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
@@ -310,7 +310,10 @@ static Error executeObjcopy(CopyConfig &Config) {
       if (Error E = executeObjcopyOnArchive(Config, *Ar))
         return E;
     } else {
-      FileBuffer FB(Config.OutputFilename);
+      FileBuffer FB(Config.OutputFilename,
+                    Config.InputFilename != "-" &&
+                        Config.InputFilename == Config.OutputFilename,
+                    Stat.getUser(), Stat.getGroup());
       if (Error E = executeObjcopyOnBinary(Config,
                                            *BinaryOrErr.get().getBinary(), FB))
         return E;